(reminds me of bedsheet sets from Ikea)
compared to the nudibranch, the gills of a pleurobranch are on the side, and its rhinophores (the 'horns') are sort of stubbier.
This is the peroni pleurobranch.
Yah I know, it's not very attractive. There are some nicer looking peronis in the Sea Slug Forum.
Before you start thinking "that's a piece of coral! divewithsus is trying to pull my leg!", the gills are that crusty-looking white tuft (on the right side of the picture).
No idea where these belong - do you?