+1 to cynobacteria. Your nitrates amd/or phosphates are likely high. Most of the time these higher levels will come from overfeeding and/or your skimmer is not working right. Low water flow also helps this stuff grow
would the water change not help reslove the problem? i saw someone state that it can be do to high nitrates and phosphate. i also saw some one say low flow. i have 2 koralia evolutions. 1 is a 1050 another is a 750 is that enough for a 55