What is the white foam that boils to the top when cooking corned beef?

It is mainly the scum, a mixture of the fat and the juices around the meat if you don't wash it first.

You have two alternatives,

a) bring to the boil and drain and rinse the pot and beef, refill and reboil, and then simmer.

b) reduce heat and skim of scum.

I prefer (a) that way when you cook your spuds, cabbage and carrots in the same water will not take on any scum you may leave.

Melting beef fat.