How thick is it? Is it coming from both nostrils?
In my opinion if it was everytime you ride, I would get the vet out for a listen. Or you could do it yourself with a stethoscope. I know a horse at the barn I work at who has white, think, nasty, discharge that you would think the horse is sick, but she's not. The vet said it was because of her hay.
If it's a large amount of thick mucus I would think some kind of a Rhino virus (upper resp), and if it is accompanied by any coughing, listlesness or change in appetite I would call your vet.
If the discharge is very thin, white and ina trickle it sounds like allergy. 'Tis the season for allergies! If the horse shows no other symptoms I would tend not to be overly concerned about it. If there are other allergy synptoms there are management stepd you can take to help out your horse.
As always, if you are quite concerned, consult your vet.
It's just hard to try and tell you what it is without a picture. Could you possibly post a picture. Here's what I think anyways. It's either an upper respiratory infection or as whitefoot stated, it could be her hay. Does it smell bad?? That's also an indication of infection.