Memory quickly overloaded, screen flickering white, fan running

You have 2 major issues with your computer, first you have disabled Apple's security and replaced it with the malware app CleanMyMac X.app. Please read the following:

The best antivirus, cleaning app, and overall maintenance app for Mac OS is Mac OS itself. All you need to do with Mac OS to keep it secure and running well is to keep it up-to-date and do periodic (about 1x per week) restarts. Other than that, leave it alone. Adding third party antivirus, cleaning, security and other types of maintenance apps to Mac OS adds no additional level of security. The only thing these apps do is have the opposite affect users want. They make Mac OS slow, unstable, generate odd behavior (much like you are experiencing) and make Mac OS appear buggy. 

Please locate the developers uninstall instructions for CleanMyMac and follow to the letter. Then restart in Safe Mode per the directions in Use safe mode to isolate issues with your Mac and then restart normally. 

Then you need to re-instate Apple's security. If you do not know how, please contact AppleCare ASAP!!!! AppleCare Contact Info

I am seeing symptoms of overheating. That is very possible with any activity that needs a lot of CPU and GPU resources. That includes rendering apps, gfx-intense games and sims, streaming video, and video-conferencing. All will heat up an iMac enough to kick in the fans.

However, the next thing to check after evicting CleanMyMc with extreme prejudice is the physical positioning of you iMac.:

• the space between the bottom of the case (where the air intakes are located) and the work surface is roughly 3 inches (75mm). That amount of space must be maintained. I have tested with both temp-monitoring software and physical, remote-read digital thermometers: stacking almost anything between an iMac case and the work surface will raise internal and exhaust-air temps.
• The air intakes get dusty and must be cleaned. However, do not use anything that pushes the dirt farther into the computer, like canned air. Every 6-8 months (I live in a dusty environment) I place the computer face-down on a table. I use a non-static brush (I use wooden-handled, natural-bristle pastry brushes) and Dear Wife's vacuum cleaner set on high. I hold the vacuum nozzle about an inch from the intake vents and use the brush to flick the dirt toward the nozzle.
• Do not tip the iMac screen to its fully forward position. That greatly reduces the space into which air exhausts and temps wiil rise, even at idle. Again, I have confirmed that with two different measurement methods.

About RAM: Yes, it is OK to add RAM but it is not the cure-all that RAM vendors continue to parrot. Please make sure you are not stressing over something that changed over seven years ago.

Starting with macOS 10.9 in Oct 2013, Apple re-engineered its RAM allocation scheme based on the principle that “unused RAM is wasted RAM.” "Free RAM" may be good for Windows but the current metrics for Mac RAM evaluation changed in 2013 to "Memory Pressure" and "Swap Used." The change allowed macOS to make more efficient use of available RAM for better performance. It;s like people: i a work where humans may use only 15 percent of the brain capacity, the person who uses 20 percent has an advantage!

The new metrics: If Pressure is in the green and Swap Used is zero or nearly so, everything is working normally. Used and Free are outdated RAM concepts for Macs running OS 10.9 or higher.

This screenie is from an older iMac after updating to OS 10.9 and the new RAM scheme. Note how much of its installed RAM was being used (11.1GB of 12GB), yet the computer was running perfectly.

And, sampled while typing this, here is my current iMac 5K running Mojave with 16 GB RAM:

Again, it is being well-behaved. Apple's new RAM scheme is very, very smart.

Here is an Apple support article on the subject: View memory usage in Activity Monitor on Mac - Apple Support

And another: Check if your Mac needs more RAM in Activity Monitor - Apple Support

The propeller-head version is here: https://developer.apple.com/library/archive/documentation/Performance/Conceptual/ManagingMemory/ManagingMemory.html#//apple_ref/doc/uid/10000160-SW1

Hey BabsKubin,

We understand that when using your iMac, you are noticing that the memory is being fully used, the fan is running, along with the screen sometimes flickering white. Can you tell us which app or apps are near the top of the window in Activity Monitor > Memory tab when you notice the memory is being used fully? If you notice it tends to be one app, then you can quit that app to see if that helps. You'll also want to make sure installed apps are all updated as well: Use the App Store to update apps on Mac

This article has some good information about fans: About fans and fan noise in your Apple product If you are using a lot of memory or CPU from an app that is running heavily, then it can be expected for the fan of your device to be running.

Take care!