Is it safe to delete Xcode build data?
Rebuilds automatically Yes.
Intermediate build products and indexes. Xcode rebuilds them on your next build.
You can remove it while Xcode is open. ClearMac reminds you first, in case a build is running.
Where it is
~/Library/Developer/Xcode/DerivedData
What “Rebuilds automatically” means
The app or tool that made it recreates it by itself the next time it's needed. Removing it only costs a little time. ClearMac pre-selects these items.
How ClearMac handles it
ClearMac pre-selects it in a cleanup and moves it to the Trash, so you can put it back until you empty the Trash.
This page is generated from the same rule ClearMac uses on your Mac (xcode.derived-data), so the app and this page always say the same thing.