Actually, I'm feeling like that's a new cane...not necissarily an entire new plant (i.e. keiki). This is just how dendrobiums produce new growth. Keikis are generally produced along a cane, rather than at the bottom at the rhizome (I believe). You could always seperate it later on by performing a division (and essentially end up with the same concept as a keiki - two identical plants), but I would wait until you get more canes and the plant is larger.