Moisture in skylight over shower

You have a leak. Had the same problem, messed with it for a while, had it removed and resealed, and so on. Finally had the outside dome replaced, and problem solved. Old one had harden due to lots of sum and weather, and had cracks around the edge that would not seal. Replacement solved the problem. Was not that expensive, although I would have been a lot better off if we had done that first.