Looks like Black Beard Algae (BBA) to me. A detailed thread on it is What exactly causes BBA?. I use to get find them in areas that get light on them and nothing else growing in area or in hidden parts of scape you don't normally see until doing a big WC after a big trim of plants. You can spot treat them with Liquid carbon when doing a water change (WC) if area is above water level easy, it then dies and shrimp eat it. I found getting them with tweezers easy also, although they do grip to hardscape well.
A little found in a mature tank here and there is normal IMO