FWIW, I've had good success with ordinary bleach in a 50:50 ration to
water. I spray it on, wait a bit and scrape the residue off. For tougher
critters, I use it full strength. I believe it's less caustic than Oxaloc
acid and certainly less-harmful on the lungs. It also costs less and is
far more available than acids.

Whichever you use, make sure to rinse the fiberglass well with water to
remove any residue as well as wear protective gear (I wear an old dive
mask to protect the eyes and a disposable tyvex for the rest of me).
Finally, it seems to me that removing the growth is easer when the growth
is wet - don't let it dry or it'll be twice as hard to remove.