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I have this nice red grape kelp macro algae and over the last 2 weeks this hair-like stuff has grown on it, its no where else in the tank-just on the algae.

Looks kinda like small anemones or something

http://i1148.photobucket.com/albums/...312_171039.jpg

if you look at the center of the pic you can see what they look like-small tubes with hairs.

http://i1148.photobucket.com/albums/...312_170950.jpg

digitate hydroids....not good;)

Should I do anything about them or will they just vanish over time?

I'd remove the portions of the algae that are affected. You don't want hydroids in the system!