You may find some luck with h202. If it were my piece in my tank. Key words being MY TANK =D. I would turn off all water current and wait for things to be still. Then I would take 1, maybe 2ML of h202 and gently blow it directly onto the algae. Wait about 3-5 minutes then turn the pumps back on. Rinse and repeat every 2 days until the algae is gone.
I cannot recommend YOU doing it without first educating your self on the effects of h202 in the reef if you were not already aware.