My tank had a lot of green slime algae, what would help kill it? I've got a sand sifting star in there now but it doesn't do much. LFS says I need a yellow tang or a foxface, they keep telling me the octo won't attack it but I've got doubts about that. I'll try getting a picture of it soon.

Thanks.

Any lfs that tries to sell you a fish to put in the octo tank is trying to sell you a dead fish. Period.
You have an imbalance in your tank water...check everything. Green slime algae can come from a variety of sources, usually too much nitrate...just yank it out and do a water change.