The biggest pests you can get into this hobby are montipora eating nudibranchs and zoanthus eating nudibranchs.Yours doent look like none of the bad ones and its not an aeolid nudibranch.The pest nudibranchs have a cerata similar to the one in the picture,that look like the spikes on an urchin back.
I'd still like to figure out what this is. I had a toadstool leather mysteriously die a number of months ago, looked like it was being eaten. Never added another leather after that because I could never figure out what was going on. Also had a gorgonian basically disappear overnight once. I'm wondering if this thing could be a pest.