Mogurnda mogurnda is a freshwater fish, but like other freshwater Eleotridae I'm sure it'll tolerate low-end brackish conditions just fine.
They are opportunistic predators, like most Eleotridae, so yes, they'll catch and eat anything they can. I'd have assumed adult horseface loaches would be too big and too fast for them, especially in tanks with a sandy substrate into which the loaches can burrow.
Cheers, Neale