White slime-like, possible like mucous, buildup on my mouth during sports? I maintain hydrated, so what else could be the cause?

Post nasal drip? Does this interfere with exercise? Is there shortness of breath? Cough? If not, another possibility is that during your exercise, running, jumping or core-based exercise is causing stomach contents to splash up your esophagus; this irritation can trigger mucus build up in the back of your throat. Try watching what you eat or drink just before exercise. Avoid tea, lemon water and acidic drinks.