Try using a solution of approximately a teaspoonful of bicarbonate of soda
into a cup of water, and spray the solution on the powdery mould. I don't
believe it can harm the fig tree; I've used it on my Jade Plant when it
sported a serious moldy infection of some sort a few years ago with no ill
effect, and it solved the mould infection. I've sprayed a stronger solution
of baking soda to eradicate powdery mildew on young squash plants in my veg
garden, and had excellent results. I should think the baking soda solution
should solve the mould infection as well. The beauty of baking soda is its
low cost, plus my experience with its effectiveness. . . although it may
take a few days, and a few appilcations of the solution (perhaps a little
stonger?), depending on severity of infection.
The suggestion regarding full sun for the tree is valid. And you may wish
to view some information from
http://www.ipm.ucdavis.edu/PNG/r261100211.html for much more info about
infections of figs.