Tradescantia of some kind maybe? I found a seedling from one of mine coming up in another pot. I'd yanked it up before I realized what it was. I don't ever recall having one up from a seed before, but they can grow roots from a broken stem easily enough. The other thing it looks like - except for the variegation - is a Canna seedling. I have a bunch of those now, but not from a variegated type.
