That's red slime not coraline. You prolly have an overabundance of nutrients in your system (nitrates and phosphates). Scrub it off and do a pwc and test your parameters. If your nitrate is above 20 and your phosphate is above 1 do another water change every 2 days until it is back to a good level. Also think about what could be the cause of nutrient buildup. Could be lack of water changes, nutrients in your tap water if your not using rodi water, or high phosphates from overfeeding.