Water has got into the uncured base resin system and has contaminated the cure, now the surface has white milky like appearance.

If any GRP roofing system comes into contact with water it will never cure, if it starts raining and you do not cover white spots will appear in the chopped strand matting.

If you manage to catch it in time then you can sand back and reapply the base resin or topcoat, you could also apply another layer of chopped strand matting and base resin and then topcoat.