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I know some slugs like blue neon are bad to have because all they eat is flatworm and you really don't want those...... do some research..... maybe LiveAquaria.com / slugs may help.
There are literally thousands of different kind of slugs. It's very difficult to know whether it's beneficial or not.
I would keep an eye on it and see what it does.
Here is a partial species list you can browse.... http://www.seaslugforum.net/species.htm
"Sea slug is a common name used for several different groups of saltwater snails that either lack a shell or have only an internal shell. It is a paraphyletic name used for various lineages of marinegastropodmolluscs that are either not conchiferous (shell-bearing) or appear not to be.
The phrase "sea slug" is often applied to nudibranchs (many members of which are colorful and are a noticeable part of the underwater fauna), sea hares, the sacoglossans, various families of bubble snails (Cephalaspidea), the sorbeoconch family Pterotracheoidea, the pulmonate (air-breathing) sea slug family Onchidiidae, and others. Sea cucumbers are also sometimes included as sea slugs.[1][2]"