Cyno bacteria ... there are a couple ways to get rid of it remove the rock scrub it off , scrub it off in the tank and siphon it off , increase water changes , and change filter media . You will probably need to also increase H20 flow to that area ... IF that does not work you could use chemi clean .. But you need to find the source of it . Commonly it comes from over feeding your tank .....I get it every now and then when I over feed or just need to , by removing it doing a water change lightning up on the feedings , changing out the filter media it goes away ...