Hi everyone I have a 40 gallon breeder with crushed coral that's been up and running for 6 months. And about a month ago I started seeing this bluish /black algea What is it, should I be worried? I was planning on removing the crushed coral. And replace it with sand. My cal is in check at 440 my alkalinity is at 9ish(depending on what test I use). Any ideas and help. Salinity is 1.026. Thanks in advance!