Does anyone know what this slug thing Is?
 

I found this picture on the internet but just wondering if anyone knows what this pest is and is it bad or safe? Never seen this thing ever in my years of reefing. I can't find any info on it on Google either. Thanks

Looks like a Polyclad Flatworm. Not sure if it is safe or if it is parasitic. Have you seen it on the coral tissue?

Nudibranch
 

That 100% is a euphyllia eating nudibranch

Maybe 100% euphyllia eating Flatworm.

Doesn't look like a nudibranch. Flatworm.

The only flatworms I've come across that weren't harmful were those little brown/red ones. Those can become unsightly and indirectly can cause issues.

But my guess would be it's a pest.

You're right
 

Lol it was early. I meant Flatworm.

Had recently seen them in a clients tank

Weren't easy to get rid of

Which is a Polyclad Flatworm.

There are hundreds of different flatworms some are only a nuisance and some are parasitic.

Unfortunately where there is one there are more. You often do not see them until there are many.

I do not know a lot about this particular pest. You may have to treat the tank because you likely won't be able to completely eliminate the problem with dipping now that the flatworm is in your tank. I do not know if they are host specific. Not sure if Flatworm Exit would work on it, some say it does. "Purge" might be another option.

I
 

I saw them in a full reef. Pulled anything euphyllia and they all died off

Years back I found a huge ployclad that came in on some liverock. It was about 2" long and I believe was a predator of clams! That was not good...