I would take the rock out and give it a good rinse/scrub. Rinse it in buckets of water before putting it back in the tank. Then do a massive water change, as much as you can. Siphon all the gack out of the sump, thoroughly clean your skimmer. Then get everything going again and go from there.
What does the algae look like specifically? Fuzzy, stringy, bubbly, slimy etc?
Christy
