It is an internal parasite. I'll copy some info from a reefkeeping mag article by Ron Shimek below. (I just got 12 pepermints for a friend and I and 2 had this exact thing. Dealer said htey had never heard of it but gave a refund for the two. Don't know if i should put them in with the others. Tell me what you do)
http://reefkeeping.com/issues/2002-05/rs/index.htm
(see link about halfway down for a pic)
Figure 2. Female Epicaridean isopods, Bopyroides hippolytes on the sides of two candy striped shrimp, Lebbeus grandimanus, from the N. E. Pacific. The large white lump on each animal's left side is the female parasitic isopod fastened in the shrimp's gill chamber sucking its blood.
The rather odd epicaridean isopods are rarely, but regularly, reported from aquaria. "Epi" means "on" and "carid" means shrimp, so these animals are aptly named, as they are isopods most frequently seen as parasites on shrimp. The males are very small, look like normal isopods, and are almost never seen. The parasitic females may be quite large, and look like large lumps or tumors found on the side of a shrimp. The female lives fastened onto her host's gills or upper leg segments and sucks the host's blood. It lives under the carapace, which becomes deformed over the parasite. The parasite is generally not recognizable as an isopod, but rather looks like a large white lump on the side of a shrimp.
Epicarids appear to be rather well-adapted parasites, and do not seem to harm their host much, in spite of their rather ghastly appearance. They may be found occasionally on the sides of all of the various shrimp imported for the hobby. They appear to be most frequently seen in peppermint shrimp, Lysmata wurdemanni.