As Orion metioned, it's really hard to assess what it is without context. IME, BTAs tend to be messy. What I mean by this is that they spew discharge from time to time for no apparent reason. What you photographed could be waste or zoox. I'm not too concerned because it's stringy, which leads me to believe it's waste. Either way, it could be that the nem is processing old zoox to make way for new growth.
They also deflate and ball up for no reason, more than other nems I've had in the past. In short though, I wouldn't worry about it unless it remains deflated for more than a day.
How long have you had it? Is it adjusting to new lighting? What are your water parameters?