Hard to tell without leaves, bark looks similar to what i have, which is ficus microcarpa (otherwise known as tiger bark fig/ginseng fig and a bunch of other common names)
Even with leaves it may be difficult to ID Ben. Most ficus have quite similar leaf shape and colour. Unless it is one of the ones that has unique leaf shape/texture the only sure way to ID ficus is from the fruit they produce and bonsai figs rarely produce fruit.
ID is not crucial to growing ficus bonsai. Most of the species like similar conditions and respond to similar pruning, etc so it does not really matter what species it is.