Hypnea in a large enough clump will eventually just develop holdfasts on rocks if the flow is low enough to keep it from blowing around.
I've had success with rubber banding Dragon's Breath(? or tongue? I always get the two confused) to live rock as long as the clump was big enough (a single frond won't cut it).
I'm still unsure if the stringy tough stuff on live rock was from Chaeto dying back and regrowing on it, or hair algae....
I've never had success with ulva, even after rubber banding it for months.
Botryocladia is still an experiment, but it seems to be handling being rubber banded to a rock quite well.
Caulerpa prolifera (my love), is fairly easy to get "rooting" onto the finer substrates (smaller pea gravel size or smaller).