1) Is it ongoing, or a one-off? As in: "I once saw a glob of what looked like jizz on her girl-parts", versus "she's got a glob of something every time I see her".
2) What's the color/texture? Chunky/clumpy, like cottage cheese? (Yeast infection, almost certainly - probably stinks, too) Clear slime? (may be nothing but her normal juices, may be problem - more info needed to decide) Looks like actual jizz? etc.
3) Is she in heat - I assume, since you're horse-sitting, that you're qualified to tell?
3A) Is she turned out with a male (with or without balls)?
4) Is there a foul smell? (rotting meat/fishy/moldy/yeasty)
5) Any signs of blood in the whatever-it-is you're seeing? If yes, get with a vet ASAP and straight to hell with what might get reported - the problem may be life-threatening serious.
Getting much more specific than that needs a bunch more information, and may require hands-on inspection, (and to the inevitable wankers, no, I'm not talking about "lemme come play with your mare", so put your dicks back in your pants) sample collection, culture and look at it through the microscope, and so on.
*IN GENERAL*, a mare is pretty well "self-cleaning" back there, and if the discharge isn't "nasty" (either visually or stinky) and she doesn't show a fever or other signs of being sick, it's *PROBABLY* nothing significant.