Strange blue grey spots/mild afte winter storage

[PvL]

We have our 22 cruiser out of winter storage and are doing the usual spring clean up/ service stuff. We had the boat covered in a large blue tarp and it seemed to come through winter just fine. However I am noticing blue grey spots on one side of the boat, the starboard side. At first I thought it was mould but after washing the boat, I am not so sure. Could it be some residue from the tarp?. It does come off with a ton of elbow grease . Has anyone experienced this and what did you use to clean it off with?

[potter water]

I've use clorox cleaner that comes in a spray bottle to take care of the occasional discoloration that you get on white gel coat. Usually from a rotting leaf that sits on the boat, bird doo doo and rust marks. It works well and seems to have little affect on the gel coat.