Its hard to tell in the pic (needs to be closer) but it kind of looks like a zoo eater. Regardless you are better off getting rid of it. Its definitely not an algae eater.
Start inspecting your zoas if you've noticed them disappearing. Zoa eating nudis camouflage themselves, turning the same color(s) as the zoas they're eating. Look carefully with a magnifying glass before your lights come on, you'll be able to see them easier.
Dipping in Lugol's/Revive/Coral Rx/FW will help to kill the adult nudis, but not their eggs (chances are, if you have adult nudis, you've got some eggs- they look like little white spirals on the zoas and need to be scraped off). I'm currently investigating a suspicious nudi situation in my tank as well.
regular temp, ph adjusted freshwater dip will knock them off but wont do anything to the egg's, get a wrasse in the halichorus species they will constantly be searching for them or anything else alive in there to eat. btw if you see egg's they are pretty easy to scrape off
Do you have a wrasse in your tank? Coris wrasse are pretty good natural predators of nudis. They will not get rid of them all but they do help. You will need to check for more nudis and especially eggs and manually remove them.
So figuring from all these post this nudi is a pest. Nope i do not have any wrasse in my tank as I do not have a cover. I did two wrasse in the past and they went carpet surfing. Any wrasse out there that do not carpet surf?
I don't want to Jinx myself but I had a yellow wrasse that helped take care of my Monti eating nudibranch problem and along with a melanarus did really good finishing off every one of them. My tank is open top too.