mealy bug

Those are definitely nasty little mealy bugs. They will suck the life out of your banana in time. They reproduce like wildfire too and are difficult to rid. It is best to cut off all the infected leaves and throw them away, not to the compost. If the banana is growing in a container repot it in fresh soil. If you have two leaves that are touching, cut off one. Where it is dark, warm and moist the bugs will live and prosper. Keep the leaves far enough apart that you will havve ample air flow. Hope this helps.