Am I overcooking whitebait in water by using scum as a measure for doneness?
Substantial visible albumin is generally a sign of overcooking. As fish cooks, it contracts and squeezes out protein (and moisture), so if you've done a lot of that, you're heading in the direction of dry, tight fish, not tender, flaky fish.

This should be obvious from eating the fish; I doubt it was fully tender and flaky. It's also obvious from what you describe: it took five minutes, which is a pretty long time for fish that small. I wouldn't be surprised if your fish were pretty much at the water's temperature after five minutes.

As for your argument about temperature, well, yes, albumin is released in the 140-150F range, and that's pretty close to the "done" temperature.

But it takes some time for protein to work its way out of the fish into the water; it's not like it all jumps out at once as soon as it hits a high enough temperature. Cooking in water delays things even more: the protein gets diluted, and it takes takes time for it to all get to the top and collect into scum, so it might take even more released protein for it to be visible. During that time, the temperature of the fish is still increasing.

Bottom line, if you're unsure how done is done enough, test the fish. Use a thermometer, or just flake it open with a fork or knife to see the texture.