It could be the 'web' a Vermetid Snail (that really doesn't look like a snail at all) extends when feeding. Usually the snail draws the web back in and eats whatever is attached to the 'web'.
thanks for your reply Kathy , got home from work today and found it around some polyps and from a rock to the tank wall , is this harmful ? is there something that eats this? looks like mucus
