Larry,
If it's "fiberglass" consider posting a question in automotive repair about painting fiberglass.
If it was me, I would stop the waterfall, let it dry, sand the area with a shop-vac sucking away the particles at the same time, then after wiping it clean, a coat or two of automotive type touch-up paint with adequate drying time before second coat and/or letting water run over it again.
I don't have a fiberglass waterfall for
my pond.