Joker is claiming he's smoking good weed and Batman disagrees telling him it's low quality. Then Joker goes to clean out the bottom part of the grinder he uses to prepare his weed, which is the part that catches all the tiny weed particles, so it's the most potent part.
Also people tend to leave their grinders dirty for like, way too fucking Long and let the keif build up on the actual screen and window/middle bits where the weed actually sits, so rinsing ANY part of it is a waste.
My mother has brought over her grinder and it's so fucked I can barely unscrew it, have to spend 10 minutes scraping that golden beauty off the sides of her grinder (end up with $40 worth at a dispensary lol) before I can alcohol bathe it
Also if you rinse your grinder with water your going to end up so sticky lol