If it were my tank, I'd kill it off. Unless you know for sure that it's a GOOD GUY, it's not worth risking everything else in your tank over.
Naper, I started with one majano in my tank a couple years ago, and thought what you did. Everything I read said they were mostly harmless, so I just let it be. At this point, I have hunfreds of majanos in my tank. They've managed to spread across 7 feet of rock. They have started growing on top of my corals, and have killed off several of them. Everything they touch turns white and dies.
I do not recommend leaving even one majano or aiptasia in a tank just to "see how they do".