Eric
I realize you do not have new rock but, check this out
IMO It's die-off: rotting dead organic material like sponges, macroalgaes, etc.
White areas on new
liverock that is going through the
cycling process appear that can look like "cobwebs" or "fungus". This is due to
benthic organisms, such as
sponges,
corals or
ascidians, dying. The death is likely to be due to the stress during the shipping of the
liverock (which the hobbyist has no control over) or due to the high levels of
ammonia /
nitrite during the
cycling process (which the hobbyist has control over).
Bacteria most likely. You could take the rock out and scrub it off in some used tank water. Or syphon it out.
Might check your water parameters etc. I would spend some time give the tank and rock some scrubbing love
GL
Sully