Begonia cf. "U577" is a species from Vietnam, initially introduced into cultivation in 2009. It is an interesting species with showy mostly greenish foliage with silvery-lime central splashes, and a creeping habit. The plant seems to form a small rosette which sends out runner-like stems that root down as they grow - which also makes for easy propagation. Clipping the tips back encourages the base rosette to get bushier.
These are 95% quite likely to be the same as U577, however, I have yet to bloom and/or have their identity confirmed. Until that time, one should continue to use: Begonia cf. "U577"
These are established plants, currently in 3" pots.