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It certainly looks like mold, but that doesn't mean that it's growing from inside the core or the cabin and spreading to the outside. Mold spores could simply land on the cabin top and grow from there.
The only real way to know the condition of the decks and cabin top are to get a moisture meter and read the content of the core. At the very least, you can walk around topside, and feel for spots that give way under your weight, and you can gently tap around with a screwdriver handle or a phenolic hammer and listen for sound changes.
Sailingdog has written a buyer's checklist around here somewhere that will tell you what to look for. Use the site search engine to find it, it's mentioned about 100 times.
Alacrity, 1981 Tartan 33 #168