This is a stab in the dark but it has hapoened to me, so i'll put it in the mix to cover all bases.
In the past when i've sawn up egg crate/lighting diffuser to snuggly fit somewhere, unless you saw it cleanly you can be left with a very rough "bitty" edge. If you don't smooth these bits off they will, over time come away. Lighting diffuser is typically made from polystyrene, which sinks in water. I see you have white lighting diffuser.
Get some of the white bits and rub them between your fore finger and thumb. If they're hard and bitty, then I am a complete and utter genius. If they're just slimy and smooth then I am a complete and utter fool. Lol.