nudibranch in my Zoos--safe?

[marlboro man]

I recently purchased Zoo rock and I noticed some nudibranches living among the button polys.

These creatures take almost the same shape and colour as the zoos.
They've got spikes on their back which resembles the edges of button polyps and about 1/2 a cm in length.

Just wondering are they symbiotic or are they eating my zoos?
take them out?

thanks