FLIP-
If it's growing inside the cabin on the roof top, you 've got way to much moisture in there!
First, I'll have to admit, I've neveer heard of or experienced such a thing on this site, except for clean moisture when people are sleeping in the cabin and / or a unvented combustion heater is being used inside the cabin.
Is there moisture inside the cabin from a leak to the rain water?
Such a leak would run downward into the cabin sole, and, then, during the day, heat from the sunshiine (we occasionally have some here in California) would evaporate some of the water, then it would cool on the first place to cool inside at night, which just might be the roof.
First thing I'd do is check for the leak. There may be more water in there than you're aware of.
Recently, some new boats have come home with leaking porous joints where the top and bottom haves of the hull are joined. Cathy Mae and Steve even got a new boat out of it!
Make sure the v-berth cushions and the cabin cushions are not mildewing.
I'd air it out and put an electric boat heater (or just an electric heater set to a low setting) placed down low in the cabin to drive the humidity out.
You'll have to vent the cabin a very small amount to give the moisture somewhere to go. Leave this set up on to keep things dry until you have figured out the cause and corrected it.
There are mildew controlling sprays that kill mold spores as well as bleach the material being sprayed, both made for marine use as well as general household use, but the Chlorox Bleach should really do it.
Are you sure it's mold? Could it be a algae (blue-green perhaps) that is growing on the roof with light coming through the windows and the moisture from inside?
Is the roof painted with Zolatone paint inside? It's a mixure of two different types of paints that gives a splattered two- tone effect and hides all the irregularities in the inside fiberglass layup surface which is usually somewhat rough. Could it be a different type of paint if not Zolatone that is yielding some residue as it ages?
Who did you buy the boat from? The Factory? Gunther's Outboards? The Outbboard Motor Mart? They also have an obligation to you, and the hull does have a warranty, 5 years I believe.
Will post more as it comes to mind!
Good Luck and keep us posted!
(This is my 1000th post!)
Joe.