Matt: Is this cooked or uncooked beet juice?
You could use it in soup if cooked. If uncooked, mix it with carrot juice
and drink it. Personally I wouldn't want to use it straight as a base for
cooking grains, but would probably mix it with vegetable broth, and of
course, some garlic.
The white scum means it probably is on its way to spoiling, you were right
to toss it out.