Hazardous materials while breaking down

my wife and son told me the white grease like material under the heat sinks on boards is bad stuff, any one know?

I'm no expert but I've been handling heat sink compound for over 20 years with no ill effects I know of. What's wrong with me now is not related to that stuff. I used to do electronic radio repair and some days I'd be in that jar of compound all day long. I wouldn't use it in place of toothpaste but I don't believe it will harm you. Just wipe it off with a rag when your done.