cobweb

Anyone have experience with this stuff? About 3 days ago I noticed this white cobweb looking stuff growing on one area of a certain rock, since then it has spread to almost 1/3 of the rock. Started out real thin and loose, pretty unnoticeable, but all of a sudden it has started getting thicker...

So today a cobweb like fluff has appeared I am unsure whether it is good or bad. I have trochus snails which have bred before which is a possibility. I have a pair of clowns, lawnmower blenny and Diamondback goby. I did have a big conch and nassarus snails but they never come out I don't know if...