Floating Anemo Slimes are found at several locations in Teyvat. These Anemo Slimes are harmless and can be seen floating around trees, columns, etc., and are usually found in groups. Popping all Floating Anemo Slimes at one location will reveal a Chest.
See the Teyvat Interactive Map for locations.
As of Version 2.7, there are currently 12 locations throughout the land of Teyvat where Floating Anemo Slimes can be found.