Nonskid surfaces are notorious for holding and breeding mildew and mold, esp in a damp climate and with neglect. Generally a vigourous scrub, or in extreme cases, a gentle pressure washer will take care of it.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)