I think you need to target feed that animal. It's tentacles are very short which indicates it has not been getting sufficient light. How long have you had it?
First off brine is about as nutritional to your fish/nem as a pack of gum is to you. Try target feeding mysis shrimp they are much more nutritional and can be bought at about any fish store in little cubes for about $7. What type of lighting are you using, and how old/ what size is the tank? That nem to me looks like it is consuming itself to stay alive. It should have much longer and far more compact tentacles. Even if it was bubbled up it wouldn't look like it does in those pics. Provide us with some more info and we can help you get it healthy again.
Are you using the stock lighting on the biocube? It may not be strong enough to keep the nem. I've heard several say they have had to add lighting to theirs. I'm just throwing out ideas now. What are your parameters? And what are you using as flow? Something is off, if you've had it a month and it still looks like that being fed. Try feeding it more often say 3-4 times a week till its tentacles grow back.
No need to get offensive. But I'll say it one more time its either something in the water parameters or your lighting. A healthy bta will take as much light as it can get. Two of mine sit directly under my 250w mh and still eat mysis like crazy. They hide from it for just a few reasons. 1. There sick (probably not the case here as the mouth is closed and its not deflating) 2. new addition trying to acclimate but usually move back to bright light very soon.(also not the case here because you have had it for a month) 3. They don't like the lighting they are receiving.( most likely the issue) if they don't like the spectrum they will hide from it. In this case it's roaming I think trying to find more and better lighting. Again the nem looks like its consuming itself for energy hense the very short tentacles. Add Some reef capable leds or a t5 or small Mh and I bet it will be a lot better
Just a question did it look like that when you bought it?