That's the inlet diffuser. It's on the input side of the filter (from pump). It wouldn't be the source of a suction leak because it is on the return side of the pump. The fact that it moves is not really a big deal and likely designed that way so you can remove the white PVC elbow easily if the bulkhead fittings needed to be replaced. With that tube glued on, you would have limited mobility and clearance.
Suction side leaks would only be found
before the inlet of the pump, not after it.
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DE blowback is caused by either the grid array not properly seated or connected to the outlet port (check the o-ring, part #17 in this diagram
https://www.hayward-pool.com/pdf/manuals/ProGrid-DExx20.pdf) or because of a tear in the fabric on the grids.