It looks like a booger splat when your really sick =0) Need better focused pictures. I've yet to hear about a neon green nudi hitchhiking or a neon green slug though. Someone should chime in with what they think it might be but don't get your hopes up until you can provide better pics.
Did you take it out? It should have been your priority if you were able to get a pic of it. Check out melevs site for hitchikers too. might get yourself some quick results there.
very hard to tell from the pics, but looks like a nudibranch of some kind...b/c of the green color I would guess a Zoa eating nudi...but as mentioned above, nearly impossible to ID without a better picture
Where did you find it? On coral, glass, algae, etc.
I am just guessing, but based on the dark green color of the whole nudi, I think it eats algae. You must look and find out, but if I had to guess its a algae eating nudi. It has that chloroplasty green color.
Zoanthid eating Nudibranch can change color to be the same as the zoanthid they are eating.