Doesn't matter much what medium you have, a small seed can hide just about anywhere, think of a dandelion. Heck, you could of even carried it into your grow room on your body. You have struck my curiosity though when you say it looks like a MJ seedling, does it just have cotyledons or does it have true leaves? How old are your plants? If it didn't come up with your seedlings I'm guessing it wasn't in your medium before hand. There is a species of tree that grows new trees from the root system. There are whole forests of them, above ground it looks like hundreds of trees, but they are all part of 1 plant. Never heard of an MJ plant doing this though. If your worried, just pull it up. Or you can let it grow out a little and see what it is.