Your description sounds like cyano. It is normally in every tank and is not harmful. It forms visible (ugly) mats under certain conditions that are not well understood (despite endless speculation on threads) but in general are more common in new tanks. It is easy to suck out the mats with water changes if you want. Or you can blow them off rocks with a turkey baster and filter out with filter socks. You should invest some time to figure out how to take good pics of your tank; it will be very helpful every time you post a question.