Haha I think you are all over reacting.

It's spring. It's that time of the year. The mare's in heat - it's normal discharge.

I actually went on a ranch call yesterday, and the client asked THE SAME THING - she had 2 mares with whitish discharge from their vulvas. Her employees were freaking out, telling her that her mares needed to be flushed and this, that, and the other...

Unless it's bloody, don't worry about it.