I would use chemiclean . That's the only way I got rid of it . It started on a frag and thought it would go away a week later my whole rock had it . I did numerous water changes and scrub the rock but three days later it would pop right back up . Until I used chemiclean it went away ever since I haven't had any problem with green algae.
Chemical treatments work, but you still have to solve the source of the problem. Make sure nothing has died in your tank and you're not overfeeding. Best of luck.
It's likely from overfeedong. Old tank cracked everything is in a temporary tank with out a skimmer. Should I treat the whole tank with chemi clean or just the one rock/coral.