You may end up having to use a chemical spray with pyrethrin on the plant. But, I would first recommend you try a natural product like BotaniGard ES. This helps keep the populations down with regular spraying. If you can take the plant outside and spray it down with a garden hose with a hose end sprayer, that will help knock them off before you start ...
University of Florida has a paper on Gynaikothrips showing that the larvae are this white yellow colour which go on to produce adults which are black and are found on Ficus benjamina. Might want to take a close look and see if you can see the black guys on your ficus and compare with the description.
Update: as well as aphids
they lay eggs on the underside of the leaves
and if you disturb them they fly away
don't use any chemical sprays as they become used to it and if only makes problems worse.
1 teaspoon liquid soap
1 litre of clean water
optional ( one drop of neem oil)
make a soapy water solution using nontoxic washing up ...