Does anyone know what this slug thing Is?

[Frogger]

Originally Posted by smokinreefer

Maybe 100% euphyllia eating Flatworm.

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.