They usually reproduce by splitting so sometimes they will sort of stay confined to one area for a while, but I definately wouldn't take any chances. They are hard to eradicate once you have a bunch of them. with just one, it's pretty easy.
I'd start by carefully trying to peel it off, if you can get all of it you're good to go. If a little shred tears off, it'll probably grow into a new manjo and you'll need to take more drastic measures.
I had one for a while.. He was cool.. I fed it.. hahaha then one day it split.. I killed its evil twin.. then he was fine for a lil bit.. then split again... so i was like ok if you cant be happy as one nem then your done.. and i killed him.. haha
You can kill them by injecting them with hot water, lemon juice, some kalk.. anything to throw off its pH.. I injected mine using a needle i bought from cvs at the pharmacy with lemon juice.. then when it died i scraped off what I could see on the rock and it never came back..