I think it’s powdery mildew. Here’s some info from Euphorbia-International. I’ve never dealt with powdery mildew so I don’t want to give advice in something I have zero clue about;).

I agree with Kara, it does look like powdery mildew. Interesting, I've never seen it on a Euphorbia before.

Same, and I too had never seen it before. Lots of air movement helps prevent it, at least on other plants.

I have personal experience with it! I had never had an obesa contract it but one morning, there it was and not much left of the plant!