Pica Pica. Too late to do anything now, right when you get it spraying with anything containing ammonia helps (windex, pee (have a friend help or you may hurt yourself), etc.).

There's been plenty of debate over what Pica Pica is but Green Reef reported it as being an algae bloom that happens to occur around the same time the thimble jellyfish appear.

Doing a search in the forums will give you more information.