Help me id this thing

[Lb9234]

HELP! I bought a 20 lb live rock that came with a bunch of mushrooms and than this morning at around 5:00 am so the lights were still off I saw this red and white creature come out looks almost like a caterpillar and has spikes....I tried to identify it but I can't seem to match the picture with anything...it doesn't have fuzzy spikes like a bristle worm I think....if you know what this is please help me, it is also round and not flat if that helps

 

[dhanking]

tiger tail cucumber

 

[Lb9234]

tiger tail cucumber

Thank you very much