I was not really trying to "get you" but went back to look at the picture (that is really a good photo of a new hatch) to see what the arms looked like when they were first born because I had no clue (and I half remembered a stripe pattern - mercs don't show a pattern like this so I noticed it when you posted). The absence of an explanation was intentionally devilish but the comparison was serious.
As I allueded to above, serpent typically only have 5 arms like most starfish so if you counted 6, keep guessing. I did try to find my night worm that looks like a serpent but never moves but I rearranged some of the rock in Puddles tank and I am afraid I moved it and can't find it now. I think the "arms" of this thing are much slimmer than what you are watching though.
Try touching an arm with a tiny piece of shrimp on a string and see if it grabs the string. Serpents can bend their arms a lot but you might be able to see more of what is under the rock.