What is simdiskimaged on a Mac?
Quick answer
simdiskimaged is the macOS service that handles simulator runtime disk images, the iOS versions Xcode's simulators run on. It runs while simulators are in use (it was running on our Mac alongside two booted simulators). It's a normal part of Xcode's simulator system; don't force quit it.
Why it's running
Simulator runtimes are disk images that get mounted under /Library/Developer/CoreSimulator/Volumes. simdiskimaged is the service involved in managing them, and simctl runtime commands work with it.
If it stays busy
Shut down simulators you aren't using (xcrun simctl shutdown all), quit Simulator and Xcode, and give it a minute. If it keeps happening, restart your Mac.
Reclaiming the space it manages
xcrun simctl runtime list
xcrun simctl runtime delete --notUsedSinceDays 180 --dry-runQuestions
Is simdiskimaged malware?
No. It belongs to Apple's simulator system, installed with Xcode.
Can I stop it permanently?
Not if you use simulators. Without Xcode's simulators installed there's nothing for it to do.