Q.Ficus benjamina aerial root?
I’ve noticed something growing and getting a little bigger about a foot from the base of the trunk on my almost 6′ tall ficus. It’s been outside all summer and has enjoyed full sun, rain and humidity for the past 3 months. It’s tripled in size. What is this growth, shown in the photo I’ve attached? Beginnings of an aerial root? I know it’s definitely not new foliage growth. Thanks!
This appears to be a Spindle Gall from this image. It would be unusual to only have one.
You would normally see clusters and they will increase. Just be aware and watch for more to form.
Here is link with more information.
If it does happen to be a side shoot as you mentioned, I would remove it.
It also wouldn't hurt to treat with Neem Oil. It can do no harm and acts as both an insecticide and fungicide.
Here is a link with more information.