On the HULL? That's one I personally haven't seen before. On the vinyl, yes, but it seems to reason that the causes would be different. On the plus side, it's likely much easier to get rid of whatever that is on the hull. Try spraying it down (mist it, out of the sun) with a mixture of bleach:water of about 1:4. Give it about 5 or 10 minutes and then mist again if needed. Adding a tiny bit of something like Dawn can help it cling to the gelcoat. White paper towels will cling to the surface, as well - concentrating the bleaching action.
On & Off hull cleaner would be my next go-to if the bleach doesn't work.
Again, though, I have never come across this. But the methods above won't hurt anything and are readily available and easy to do. Maybe someone else with actual experience with this can chime in, though?
The only other thing I can think of is some type of leftover micro-organism. The above methods should rememdy that.