Brain Problems. *More problem critters found....sigh*

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Well a bit of an update. I have been spot feeding with only my top notch food and raised my lights up to 24" above the tank as it seems the other brain I got from the LFS (long story) isnt taking any food and doesnt seem all puffy and inflated either. So I kept thinking it was lighting (see "lighting question in DIY) , water quality (tested everything again tonight and only things that seemed a little off was my phosphates were at .20 approx and my calcium was a little low at 380 but i am due for my waterchange) , food quality. Something im doing wrong. So during spot feeding a bit of mysis I noticed something very interesting. The piece of LR its near has had the sand pushed out from it. I thought it was just my tigertail moving around a bit as ive seen it in the area. HA! Not with my luck. I just got the two xanthid crabs out. Yaaa. So im totally not sure exactly what it is... I saw these antennae about 4 or so of them 1/2" to 3/4" long flickering. Looked at it and *poof* gone. Instantly I froze, watched. I saw antennae again and a claw come out and grab a small piece of mysis. It seemed a purpleish red color. From what I saw it had a hard dome head. Unfortunatley I dont really have an idea of what the front of this thing looks like all together as I only caught glimpes of it in pieces. I searched eunicid worm and it has the same kind of antennae but no claws. Looked like a crab claw but what i saw from the front quick it was no crab. Anyone have any ideas? Im guessing its going to be a pull the rock he lives in and try for a removal or smash but Id like to make sure hes the problem before I rip into the reef. Sat for an hour watching this area with a camera after and nothing. Ill have it out for feeding time tomorrow and see if I can snatch a pic. *sigh*