No, it's not something I've ever encountered growing broccoli starts indoors and I've been growing them for a couple decades and at least 10 varieties. It resembles powdery mildew,

I've experienced that with a number of brassicas over the years. Seems like a sort of wax produced by the plant that makes the leaves water repellent.

It doesn't look like powdery mildew to me - and the plants look healthy, but the leaves aren't shaped like any broccoli I've grown - could they be cabbage or collards?

It's natural and nothing to sorry about. Broccoli's ancestors are coast dwelling wild cabbages which have to resist desiccating salt spray and high winds. Google epicuticular wax for more information.