Highly doubtful it's a mantis. There are stomatopod's that can get to the size to take a juvi Naso. However, it's far more likely to be an isopod.
Secondly, if it was a spearer mantis it would be literally be hanging by one arm which most likely would tear off. I don't see a scenario where this would work.
Something like this is an Isopod:
Or:
Doubtful but here is a spearer rap:
a Mantis with/spearer:
I hope that helps. Again, doubtful if it's a Mantis. But those pics should at least point you in the right direction. Once we know what it is specifically we can help you get it off.
A picture would help a lot.