The spalting is just another form of mold. It is in the process of decaying the wood. It makes for some great figure in the wood, but it is what it is. If the wood is solid enough to turn, you can turn it. Be careful of punky areas, as they will want to blow out on you. If you do turn it, make sure you have a good respirator and dust collector going, as you don't want to breath in the mold. Lots of folks send out the spalted wood to be stabilized.
Brian