<a href=showthread.php?s=&postid=6543154#post6543154 target=_blank>Originally posted</a> by egdevilboy
im guessing the redness...
is it hard? if its hard and seems to be "encrusted" onto the rock its probably coraline algae.
if its "hairy" it may be cyano algae, which you should remove and check your nitrates and phosphates...If it doesnt spread than dont worry about it.