How do I keep fake snow on a sytrofoam ball?

It will probably shed for a bit as pieces may not have caught the glue. Try giving it a shake to get any remaining off, patch blank areas as necessary. Sealing it will make it crispy or melt it, depending on the snow material and sealer.

I think they tend to lose some of the fake snow. Try tapping off as much as you can or you can try spraying them with a clear coat like Krylon.

Spray with a clear sealer. You may need to test to ensure the sealer doesn't cause an issue with the fake snow or sstyrofoam ball.

Try spraying with a few coats of hair spray. If you make more use a spray adhesive like 3M Super 77

If you use epsom salt you just put glue (like elmers) on the ball and then roll that ball in the epsom salt. If you want you even add glitter to the epsom salt first.

https://www.instructables.com/How-To-Make-Fake-Snowballs/

You may try spraying it with a clear sealer just like Krylon.

I would either roll it in some Elmers glue let it dry completely or go to the dollar store and buy a really cheap hard to hold hair spray, that should seal it.

Either a clear spray on sealant or cheap super hold hairspray. Do the after coating outdoors.

I would spray it all over with a clear sealant. But gently tap it all over to remove any loose flakes first.