A lot of foam while cooking a cod with steam
This sounded quite normal. Steam condensing on your fish is washing small amounts of protein into the steaming water below. It is quite harmless. As mentioned in my comment above, most of us have experienced that when boiling cold eggs too quickly causing the shell to crack and some egg white to leak into the water. That always make at least a bit of foam and often enough to overflow that pan.

There are a couple of things you can do to minimise that:


pat dry the fish before steaming
use a container under the fish to catch any juices so that none of it goes into the steaming water


More water in the pan and a gentler boil might help too