I don't know if a wolf eel would be safe with a mandarin. I've only ever had one, which I still have.
I got mine quite a while ago, I think in 2004. Until about 6 months ago, it was very timid, but it has had a personality change and will actually nip at my fingers if I put them in the tank. The change was really strange. For months prior to this, the eel was not eating, and was gasping heavily on the bottom. I kept waiting for it to die, didn't have the heart to euthanize it. I thought it was old age. After a long time, it started eating again, and has acted ravenous and outgoing ever since. Before that, it was very timid. It does not attack any of its tankmates, several of which are smaller/more timid, but it is now always interested in where its next meal is coming from...
So, maybe not the best choice for your tank. I have to admit, I go for the more aggressive fish.
The file is one I saw in a photo posted by namxas or seahorsedreams if the fish only forum (they are married). If I can find the thread, I'll link. I think it may have been one of the bristletail filefish (aiptasia eaters) that turned out to be bright green.
Here's a picture I found online. Don't know if these change color or stay this color.
How about a male green bird wrasse? They are peaceful and kind of charming.
