Verdigris on copper-bottomed pan
Corrosion like that can be scrubbed off with copper polish (or, in a pinch, with salt and vinegar). The result will be bright copper, which will quickly tarnish and darken through regular use. The pan is absolutely not ruined. As with other solid metal cookware, if it isn’t literally broken in pieces then it can be scoured into a like-new state. That pan likely has centuries of use left in it.
Dunking the pan like that doesn’t sound like a problem. The upper surface of the pan comes into contact with food, after all; no reason the bottom surface can’t. I think it’s a clever technique.
The dishwasher is your real enemy here. It’ll strip away the patina and bring the surface into contact with some pretty harsh detergents, leading to that sort of corrosion. Copper should be hand-washed.