Colerpa or red microalgae in new tank?

OK, so those who have read my previous posts know this is my first go
round with a reef tank...so here's where we are at this time.

72 gallon tank, setup with sump, skimmer (currently off), 2 power
heads, and 85 pounds of live rock. It's been that way for about 3
weeks now, and has completed an ammonia/nitrate cycle (hooray). It
was setup using R/O d/i water, and I had all my measurements checked
at the LFS and everything is ready to run.

That having been said, I have noticed a growth of red hairs over a
majority of the live rock and some of the overflow box itself. Thin,
red hairs. I've discussed with the LFS and they seem to think it is
likely to be red microalgae, and that adding a couple of algae eating
organisms (snails, crabs etc) will help...but the other option they'd
mentioned is that it could be colerpa.

So the question is this...how do you tell the diff between colerpa and
red microalgae, in it's very early stages? The growth is red,
filamentous, and seems to be broadening into leaves at the very tips
of the growths. Any thoughts, before I add the snails etc?

Mitch