search for malware on imac High Sierra

Clicking on a link would not install malware. If that could happen, macOS would be awash in malware.

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

As for add-on anti-malware or add-on cleaner apps, I’d avoid those. The built-in anti-malware does well. (In one recent case, the built-in anti-malware prevented an error in add-on anti-malware from corrupting macOS itself.)

In general: Effective defenses against malware and other threats - Apple Community

I would update to newer macOS here roo, if that iMac supports it.

iMac late 2012 and newer can get to macOS 10.14, for instance.

How to download macOS - Apple Support

The low-end hard disk drive iMac models can have performance issues, and an external SSD can help there:

Why is my hard disk drive iMac so slow? - Apple Community (why, and including a link to a work-around)

You could download the free version of the Malwarebytes application, and run it. (The free version doesn't keep checking your computer automatically, but it will do a scan when you manually click Scan.)

https://www.malwarebytes.com/

There's no guarantee that this would catch 100% of all malware, but it might give you some peace of mind.

Servant of Cats wrote:

You could download the free version of…

I’d suggest not. Issues with phishing are unlikely to be resolved with add-on apps.

Adding apps intended for unrelated purposes—clicking on phishing links does not download and install apps—can introduce issues, too.

Worse, some of the add-on anti-malware apps will find and report non-existent problems, and that’s when the app is not advertising itself.

macOS is not immune to malware.

But there’s little reason to send out a phishing mail that prompts for sensitive data, if “merely” clicking on a link can itself load the malware. (It cannot.)

The initial 14-day trial of malwarebytes runs its detection daemon continuously in the background with the highest process priority of any user launched application. That's why it should be run, and if nothing found, removed via its Help menu immediately afterward.

That's generally only a problem on Windows. If you want to be completely certain, open the Downloads folder and throw away anything you don't recognize.

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The free Malwarebytes may take care of it...

https://www.malwarebytes.com/mac/