That thing with the clear lid

The strainer basket (leaf trap) is designed to facilitate the priming of the pump and suction line. It is not designed to remove air from the suction line. This trap will also work as a strainer basket to trap leaves and debris.
Stops leaves and debris from clogging you swimming pool pumps impeller.

D.E. in pool??

Why is D.E. leaking into my pool?

There are only a handful of reasons why D.E. could be leaking into the pool:
1.Too much D.E. was added.
2.There is a tear in the fabric of the grid.
3.There is a crack in the top manifold.