If the algea is black or nearly black with a slight purple or blueish hue, it is likely a species of cyano bacteria. Based on your description, it definitely sounds like cyano bacteria. In addition to mechanical removal by peeling it out, red legged hermit crabs are known to consume it as well as sea hairs and trochus snails. Luckily this type of stuff usually goes away as the tank matures. Controlling nutrient import and maximizing nutrient export (ie protein skimmer) helps also. How old are your light bulbs? As they age, they shift color to a wave length that is more friendly to cyano bacteria.