Q: malicious app

Try downloading an anti-virus software and scanning to make sure nothing was left behind. https://www.avast.com/en-us/mac or another program you would like to use.

Do not use an AV software on your Mac.

There are no viruses that can attack Mac.OS X. So, anti-virus programs are selling a cure for a disease that doesn't exist. And they are renowned for borking Mac systems.

A Mac can pick up malware but not if you just use common sense. You really have to actively invite some malware in for it to happen. That means downloading and knowingly giving your password for it to download. Mac provides all the protection you need with SIP (System Integrity Protection).I About System Integrity Protection on your Mac - Apple Support

If you do happen to download adware you can use MalwareBytes to get rid of it. Malwarebytes was developed by one of our own colleagues here in ASC. It gets rave reviews and is about the most proven anti-malware software for Mac.

Safari pop-ups are different and easier to deal with. In Safari 9.1 and later just close the window. For earlier versions force quit and hold the shift key while restarting Safari.

https://support.apple.com/en-us/HT203987

Finally, stay away from programs that purport to "clean your Mac". They can do damage to your system. Your Mac needs no cleaning in the first place. All the cleaning that is necessary is taken care routinely by OS X.

to make sure your mac is not infected by a malicious software

go to www.adwaremedic.com and it will direct you to malwarebytes.. download and install the app.

then open the malware bytes app then click scan button.

click the remove button to remove all adware installed on your mac and restart the mac.

for the fan issue follow this -->  How to Reset NVRAM on your Mac - Apple Support

if the fan issue persist even after the NVRAM reset bring it to the genius bar.

Avast is one of the worst. It is practically Malware itself, it's messed up so many systems. Search for Avast in these forums.

What app? What makes it malicious?

Malware is “malicious software,” which includes viruses, worms, trojan horses, and other programs that can harm your Mac or your privacy. Malware can be installed when you download items from email, messages, and websites.

OS X Yosemite: What is malware?

OS X has many features that help protect your Mac and your personal information from malicious software, or malware. One common way malware is distributed is by embedding it in a harmless-looking app.

You can reduce this risk by using software only from reliable sources. The settings in Security & Privacy preferences allow you to specify the sources of software installed on your Mac.

1. Choose Apple menu > System Preferences, click Security & Privacy, then click General.
2. Select the software sources you will allow to be installed:
   • Mac App Store: Allows apps only from the Mac App Store. This is the most secure setting. All the developers of apps in the Mac App Store are identified by Apple, and each app is reviewed before it’s accepted. OS X checks the app before it opens the first time to be certain it hasn’t been modified since the developer shipped it. If there’s ever a problem with an app, Apple removes it from the Mac App Store.
   • Mac App Store and identified developers: Allows apps from the Mac App Store and apps from identified developers. Although apps from outside the Mac App Store are not reviewed, the identified developers are registered with Apple. If problems occur with an app, Apple can revoke its authorization. OS X checks the app before it opens the first time to be certain it hasn’t been modified since the developer shipped it.
   • Anywhere: Allows all apps from anywhere. This is the least secure setting because the source of the app is not verified, and the app is not checked to see if it’s been modified or is broken.

In addition to apps, other types of files may or may not be safe. Scripts, web archives, and Java archives have the potential to cause harm to your system. Of course, not all files like this are unsafe, but you should exercise caution when opening any such downloaded file. An alert appears when you first try to open these files.

Sometimes you may notice that your computer fans run on high speed after you perform a data migration from another computer & spotlight begins to index your hard drive for the first time .

After you complete the data migration spotlight automatically indexes the contents of your hard drive  .

This activity may increase the CPU utilisation and the computer becomes warm .

The computer fan turns on and can make noise , runs fast to cool the computer . after spotlight completes its index the computer & its fans should return to normal idle state .

After a file has been indexed spotlight updates the index only if that file is modified .

gnetcupcake wrote:

 

Malware is “malicious software,” which includes viruses, worms, trojan horses, and other programs that can harm your Mac or your privacy. Malware can be installed when you download items from email, messages, and websites.

OS X Yosemite: What is malware?

I know what malware is. My reply was to the OP, and your reply to me has nothing to do with what I was asking. Furthermore, this isn't something you could know.

What OS version are you using?

Try a SMC reset. The article covers when this is required, but doing this will not affect anything.

Reset SMC

Try running this program and then copy and paste the output in a reply. The program was created by Etresoft, a frequent contributor.  Please use copy and paste as screen shots can be hard to read. This will show what is running on your computer. No personal information is shown.
  

Etrecheck – System Information