This will work on palms, bananas, birds, etc...

Coffee grounds. Spent ones, that is. Get a container, like Cool Whip or something with a bit of size to it and start emptying out your spent coffee grounds into it.

Put it on the infected leaves/fronds as well as around the plant, like fertilizer. A good bit too. It dries up the scale and possibly boosts the plants immunity, for lack of a better term, from what I've learned. I'm using it on my white birds, Cabbage sabals and one banana plant. I have a soap spray I use as well but that only slows it down/prevents it from spreading. The coffee grounds will actually kill it and it will eventually fall off. Unless, of course, the frond is cut off.

It doesn't happen fast, of course, so be patient. If it appears again, do it again.