pearl bubble coral

anyone knows why my bubble coral spitting out this blackish/brown sludge? Could it be the reef roids I fed two days ago?

This is my first saltwater tank setup. I switched from freshwater and wanted to become a saltwater chemist after talking to an associate who has a large set up. I currently have them in my office, on 60g cube tank and a 5g cube tank. I’m planning on using the 5gal as a display for a carpet...