I think it's called Ficus microcarpa

It is called a "Ginseng Ficus", and is actually two kinds of Ficus microcarpa grafted together. A cutting of a full sized Ficus is started, and allowed to grow large, and develop its semi-tuberous roots. When well grown, it is cut off about 10 cm above ground, replanted high, to show off the thick roots, and grafted with a dwarf variety, such as 'Green Island', or 'Golden Gate', to form a dense crown to the bizarre tree. Given time and training, it can make a really striking bonsai.