I used to have one exaxtly like that. It was a purplish/green (green was when lights were close to 10,000k and purple was when lights were closer to 20,000k and if all blue led bars were on, it kind of looked almost black) with very white tips. it was a BTA and acted just like most GBTA do, (in regards to usually lower light in a tank than an RBTA would typically go to). Sadly, it decided to move, rolled into my neon green haddoni (the massive one) and the haddoni ate it.
With that said, If a pic of the mouth and foot could be obtained, I could tell you for sure if it is a bta. Sometimes sebae come in looking like this and even bubble up a little at the tips. Not common though and the fact that the nem is in the rocks and not at the sand steers me away from even thinking of it being Sebae. I have even seen baby mags look like this, so that is even a possibility, but I doubt it.