Mystery of flatworm or sea slug?

[Sebae again]

This came on colt coral half year ago.
Does anyone know what it is? (Picture attached)

[Jason McK]

Looks like some sort of softie eating sea slug. Could even be a nudi.

[Sebae again]

Actually, I found it eating soft corals at night.
Have you had it in your tank before ?

[i have crabs]

its a type of colt eating nudi but i cant remember the name.

[Jason McK]

No I've never had one but just by the look I took a guess.