PolyMC (with one L) is the launcher that had the controversy. Prism Launcher was forked from PolyMC, and PollyMC (with 2 Ls) is a new project forked from Prism Launcher.
The PolyMC controversy is that one of the devs went rogue and kicked everyone else off the project, so they didn't really have a choice but to fork. They explain what happened on the FAQ of their website.