Ah, a member of the macrobrachium shrimp family, if I'm not mistaken.
I would separate it from the betta... Fish have been known to exist side by side for months on end, and all of a sudden an attack can occur. Keeping incompatible species is just a waiting game. And those shrimp do like to grab things with those claws, like betta tails.
I don't know if you've ever seen, but we've had several instances of ghost shrimp grabbing onto betta tails and ripping them up on here. Ghost shrimp are pretty closely related to this shrimp, so I can imagine a fish tail would be very tempting to him.