Unidentified Developer keep running in my background.

User wrote " I have tried to remove it with a third-party cleansing app like "Clean my Mac", it keeps reappearing. "

Follow the advise from Both Previous Contributors - First

Though, the situation maybe far more serious especially since " Clean my Mac" is installed and used on this computer

Any of the below should be removed as per Developers Instructions 

Disk Cleaner 

This will include CleanMyMac , aka “ BrickMyMac “

By the very aka name BrickMyMac, first coined by one of the present contributors.

This specific software can and does cause problems.

Specific to CleanMyMac aka “ BrickMyMac “

To put the CleanMyMac in context and the damages it may have or has already done.

This application can or will Muck Up your User Account ( Home Folder ) of this machine.

It does not touch the Operating System itself unless you consider your User Account ( Home Folder ) as part of the Operating System

Then in that specific context - it has Mucked Up the Operating  System

There are remedies, though they use may not like Starting Over from Scratch

Predicated on the Offending  Application has been Removed as per the Developers Specific Instruction 

Suggest restarting in Recovery Mode and choosing to Reinstall the Operating System over the existing installation. 

This may or may not replace elements of the  Home Folder ( User Account )  and replace any corrupted or removed elements and make things right.

You installed it. If you didn’t want it running in the background, you shouldn’t have installed it. Anything you install that requires an admin password is likely installing something that runs in the background. It’s always been there, running, but now Sonoma tells you it is running.

Look in /Library/LaunchAgents and /Library/LaunchDaemons. The software you installed may be running its background processes from there. It may not match what you see in the list.

If you don’t know what you installed, EtreCheck can show you.

A quick Google search indicates that "ElemntServer" is adware. ...and at least one site recommends installing IMHO a shady anti-virus app to remove it, yikes... I'd follow Barney-15E's advice and carefully manually check and remove 3rd party *Launch* items after doing a backup.