I'm assuming you're meaning macro-algae?
That's a tough one... Technically, it should be quarantined to look for any bad things, etc...
Realistically? I'm of the opinion the more variety of life the better, and just plop it in... And yeah, I know I'm opening myself up to nasties getting in, but in my opinion the greater diversity of life you get is worth that risk...
Fresh water dip would most likely kill a lot of the very life you're trying to add to a refuguim (Copepods, Amphipods, worms, etc)