He's right to a degree. Your CUC is in more danger than your fish depending on the mantis type. I've housed several smashers with fish without a problem. Not that smashers won't go after fish, but fish aren't their main prey item. Most smashers have spears as well, but don't use them much. Except for the peacock mantis, a hit to your finger may hurt like hell, but probably not split open. I've been smacked more than a few times by mantis shrimp. :facepalm:
Spearers on the other hand will almost definetely be a danger to your fish...
Either way, I'd remove it or rehouse it in your refugium. If you want an accurate identification in regards to type, show this picture in the mantis forum.
Oh yeah. Good luck trying to catch it. Their vision is among the best in the animal kingdom and they are quick and quite elusive. LOL...