I agree with Jim RO for assurance of consistancy as well as water purity.
Bottom line= if you got algea grow, you have an excess phos. source.
Prime suspects are; food added and sandbed (poss. LR if flow not high) and , of course water.
Solutions need to be cooperative. ie RO won't cure phos bound to sand in tank, just stops adding to the problem.
Personally I use my Foxface he eats virtually anything green,red etc
. the point being each fish eats algea according to its individual tastes, species usually are just a generallity.....how many of us out there have lawnmowers that eat nothing but yummy Nori?
In my tank (90g) I have a yellow tang, a foxface, a Kole and a Flameangel, all pick at different things....ie the Kole likes to leave kissy prints on the glass and Foxy picks into creveses.
Sooo add the Purple Tang, see if he eats it, add more snails, use RO, lighten feeding, syphon any sand crude you can get, harvest as much as possible.....and Good luck!
Also as a side note, I get more algea probs when my alk drops, so keep yer alk up!
I believe, as Jim says, we all battle algea at on time or another, with variable amonts of success from reestablishing balance to powerwashing your LR.
Hope this helps , sorry there seems to be no instant cure