Pie Crust Partially Cooked

What you have there is simply water seeping out of the gel and bringing some dissolved stuff with it. This is known technically as syneresis. What will help is to add something stabilize the gel. Xanthan gum is probably the easiest thing to use. You can find it at health food stores or Whole Foods because gluten-free bakers use it a lot. Start with 1/8 teaspoon pureed into your filling. Sprinkle it over the filling liquid before mixing, and put it through a sieve before baking to catch any clumps.

I'd avoid the whole reheating issue altogether and simply keep the uncooked pie in the fridge, covered in plastic wrap, then bake it as normal when ready. The only issue you might have is if your filling is very runny, in which case you could par-bake it a little before filling to help it 'seal'.