Hiya Raghnall,
That is a new plantlet called a keiki and is commonly found in Dendrobiums. I would not remove it from the mother plant as it is too small to survive independently and there is insufficient growing season for it to develop further. Better to let it drain the last bit of nutrition from the mother over the autumn/winter and then pot it up separately next spring (April earliest). You could I suppose periodically spray the roots or even wrap a little moss around the roots to help it through the winter.