big_cats, that actually sounds like hermit crab babies. I had that happen in my tank once, too. I spent a long time getting as good of a look as I could (I couldn't catch them), and hours looking on the internet and finally found
this (bottom of page 1) and
this. I thought mine were mysis too, at first, but since the hermits were right there where it was going on, and they looked exactly like what I found, I figured it out.