I've had a few pieces of timber (not mahogany) like that. The white stuff was small pieces of rot. It didn't go away. Those ukes became pizza oven fuel.
I used to do a ton of wood carving; sculptures, jewelry, and the like. I mostly used mahogany and I would see this every so often. In fact, I have some of it in my shop right now.Silica Deposits in the wood. we used to see it in Rosewood and Mahogany at the factory. the best (only) way to release it from the wood is to use muriatic acid. Good luck... and use gloves.
This happens sometimes in Mahogany, and I have heard that has to do with where the tree came from, and the soil conditions it grew in. Not much to do about it but get a different board.