very possibly a ciano outbreak as it comes in few different shades(Most forms found in saltwater range in colors from , orange-yellow to reddish-brown,blackish green to blue-green and often appear as deep purple to fully black in color)
Vacuum the affected area and find out if you are dosing some carbon(sugar,vodca,vinegar,...) base product as needs to cut down,...or maybe new sand, new rock?
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