I assume you know that it is not a stain on the inside of the wall and have taken steps to try and wash it off with bleach and water.
Then if it is just in the fiberglass just paint
it with the color to match the inside of cabinet.
I think they use chopped glass and spay the fiberglass and unless someone at the factory put somthing, like a green/black disk in this and sprayed over it there is no way it can seperate by layers.
If the camper is made, as i mke my canoes with sheet glass then a bubble can be between layers. But I do not think they do it this way.
Now this opens the question
did somebody place a sheet of fiberglass over this spot to fix a problem?
Gerry the canoebuilder