Anyone know what this is?

[Fish]

I got this as a hitchhiker on some zoanthids. It is about a centimeter long. It kinda looks like an anemone in the picture - foot on the bottom, almost transparent, tentacle things on the top. But it moves pretty quickly (relative term) and I thought it may have be a type of nudibranch.





Any help would be great! Is it trouble, or can I release it from its cottage cheese container prison?
Thanks,

- Chad