Depending on the skimmer, it can atke a week to work correctly. Give it time. I opened the air valve the entire way until it began to skim and tuned it back very slowly over the next week. Besides following X's advise, I would get in there and clean up what you already have hanging around. So, pulling the algae off your corals and taking some airline tubing and using it to siphon it out of your sandbed. I wouldn't do it all at once since that algae is currently what is keeping your nitrates down. Taking to much out at once will cause your nitrates to increase rapidly and that would be bad for your inhabitants.
Also...and I can't believe no one said anything yet...but a 55 gal
is way to small for a tang.