I've seen those! I think they're called Glo Moon rock caves, right?
I've never used them, personally - but they're made for tanks, so it should be okay - I'm not sure if your fish would mind that his hiding place is glowing, but I imagine it's dark on the inside - so no worries!
Make sure that there aren't any sharp spots that your betta could get hurt on, as with any tank ornament, and I'd say you should be good to go!
Happy tanking, and welcome to the forum!