It’s hard to tell from the picture, but there are a few things to look for to ID it as a nudi. All nudis have rhinophores on their head and their “lungs” or cerata towards the middle or back of their bodies.
If you break the word nudibranch down, you get nudi (naked or exposed) and branch (like bronchiole), or together you get “naked lung”, which is what their cerata is.
If you notice those two things, it’s a nudi of some sort. If you don’t, then it’s a slug or flatworm of some sort.