Probably not a true Munnid species (most in the hobby aren't), but definitely an isopod - you can tell by the segmentation along the shell/body - and almost certainly a Janiroidean isopod of some variety (Munnid isopods being from the taxonomic family Munnidae in the Janiroidea superfamily).
OP, these are most likely harmless/beneficial pods to keep.
Edit: Bah, I forgot some Tanaids can look like this too; OP, does the critter have little crab claw pincers, or does it have a raptorial appendage like a mantis shrimp or praying mantis? If it has the crab-like claw, then it's a Tanaid, if it has the raptorial appendage, then it's an isopod. Either way, Tanaids are generally scavengers but some may prey on worms and pods and such.
