Cottony stuff on my Live Rocks!!!!

Sorry for the exclaimation ..but there u go :-)
I saw this cottony stuff which I am guessing is
fungus on my live rocks. I scrubbed it off with
used toothbrush earlier on but the following day
it grows back, even thicker.

First of all, is this dangerous to my fish?
Secondly, how do you get rid of it?
Leave the rocks in a bucket in the sun?

Thanks for any advice!

a.c.