The Ficus benjamina is very healthy, green and full, but on the trunk there
is a growth about the size, shape, and colour of a walnut.

It is some sort of gall. It is more likely to be caused by bacteria than
insects. I would cut it off.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)