I would mask off both sides (along the tile and the fiberglass) with safe release painters tape and caulk the joint with a silicone that blends with either the tile of the fiberglass - if you cannot find a color to match or blend, then use a clear silicone. Remember that a 'skin' will appear quickly when using silicone so your working time is very short. As soon as you gun the area, the best bet is to use spit on your finger and make one clean run down the atrea. If spit is too gross for you you can use (a dishsoap mixture with water in a spray bottle) spray the area and your finger and make your run down the bead of silicone.
Remove both pieces of tape at the same time SLOWLY and always pull away from the silicone bead - don't drag or pull it or the silicone will form a ridge.
This should do it bro.