Could well be the zinc that is being sacrificed. I see the same or similar white powder from the sacrificial anode in the SS water heater in our camper. I bet if you pull one of the anodes out, you will see it already losing small chunks.
Just curious: Did you have a brass radiator in the other engine with the aluminum heads?
I use Zerax G05. It is a newer HOAT antifreeze (hybrid OAT). Similar to what was used by Mopar before their current 10 year OAT antifreeze.