It may just be pooping. Sometimes they close up, deflate (to purge/renew water.. they are mostly water). I wouldn't be overly concerned unless it has a gaping mouth (bubbling out, open), hanging limply, or no longer attached to something. If the mouth is open with stringy bits/guts hanging out.. then sound the big alarm.
My RBTA goes through phases. I was freaked out at first due to it's behavior, but as long as it gets over the behavior after a few hours or a day or so, then all is good. I also hear sometimes they need a little 'me' time. They may hide or scurry into a crevice or something for a time. If they hide and stay hidden for days or a week. Maybe time to worry.
They need some flow, quality light (mine is very happy under 2xT5HO's), and clean/stable water params. I even found if the RBTA is looking 'off'. A fresh water change can perk it up. Mine is in a 29gal tank... I typically do a 5 gal change.
Some people don't feed their nems.. but I do. Roughly ever 3 days I give a little mysis. I'm going to try small silversides (will feed in 1/4's) soon. They will squish up a bit to digest and poop. Mine poops about ~24-48 hours after a feeding usually. If it spits up some of the food, reduce feeding next time around. They know how much they can handle in a sitting.
Hope it is ok!
