As a matter of fact, I have seen this before, and it could very well be mold. Mold is one of the strangest, and least understood, life forms on this planet. Yes, some of them do look like crystalline growth (I’ve seen that, too.) A friend of mine (long gone, alas) who got me back into scale modeling, had exactly this sort of thing happen to one of his older builds. Similar paints, too.
The problem is the artist’s acrylics. Artist’s colors generally have a relatively porous surface compared to modeler’s acrylics, so both moisture and mold spores can penetrate the paint surface, and are in a surprisingly good growth environment when they do.
Assuming I’m right and this is mold: Brush off the visible stuff. Then treat the surface with a dilution of liquid Lysol™concentrate. Just mist it on in several light coats. I hope it won’t dissolve the acrylic, but Lysol has a chemical in it that is very effective on molds. The model will smell a bit odd, but that’s better than being moldy.