If you're talking PE mysis and a coldwater non-coral seahorse tank, that is pretty specific conditions and I would do whatever is suggested by seahorse keepers (I believe rinsing is in order). I even rinse that stuff before adding to my DIY reef food, it's greasy stuff. But the stuff from Brine Shrimp Direct or the like is much less to worry about, just thaw & pour off, maybe a quick rinse in RODI but that's it.