Water Seeping Under Fiberglass Shower Drain
Even a small amount of moisture is not acceptable beneath your shower pan. If the drain is properly connected you shouldn't have any seepage. If you're still getting moisture you need to determine the source. You imply that the fiberglass pan may be porous. Fiberglass isn't porous so it shouldn't be seeping but it can crack. If it's a tile shower you could also be getting leakage around the seam between the shower base and the tile. Check and caulk all of the corners and seams.
You indicated the pan looked wet underneath. Looking wet doesn't mean it is wet but you need to find out. Just take a dry paper towel and wipe the bottom of the pan. If it's wet at all the paper towel will absorb any moisture. If you're still not sure, you can purchase a moisture sensor meter online or at a hardware store that will sense moisture levels in drywall, wood and other homebuilding materials.