If you are cleaning out your Hayward PRO Grid D.E. Filter you will want to be sure that you inspect the tank belly bandO-Ring . If the belly band O-Ring is dry, cracked our split you will have to replace the belly band with a new one.
If your Hayward PRO Grid is leaking in the center of the tank, then chances are it has to do with your filters belly band O-Ring. You can inspect the belly band O-Ring by shutting down the filter system, draining the filter, and then removing the metal belly band holding the two parts of the tank together. Once you remove the belly band you can remove the top half of the filter and then you will see the o-ring. If the o-ring is in good shape then you will want to lube it and then put the filter back together.