The white powder is lead oxide, yes it is corrosion. I have used such projectiles without any issues in the past though you should take care when handling them (wash your hands, don't lick your fingers, etc.). They can be cleaned and I know I have read multiple ways to do it on this forum as the question seems to come up every so often.
I have washed them in hot soapy water with a coarse rag when I had a bunch and wanted to get them clean. Air dried then put some olive oil on them and they stayed clean for a long time.