just make sure your parameters are ok and stable. i had one hitchhike before. if they can last for 2 to 3 days on rock without water ill consider them hardy
Like any invert it can be pretty sensitive to ammonia and nitrates you may want to test for that. I've heard they are less sensitive to salinity than other inverts and they are pretty hardy. You may have just gotten a sick one, lately I have heard from several people where their mantis just started declining all of a sudden and died. Although, since yours is new I would suspect some kind of acclimation issue.
It may also be molting, they tend to do that when transferred to a new tank.
We see! called the store and the guy was like "well its been here doing great for a long time" so not very helpful!
Hope it pulls through! I am going to leave it alone tonight and see how it goes!