I did not have the stuff like around the outside edge in your picture, but the center of the diffuser looked just like yours. And on one of the six disks, was bad enough to stop flow over a considerable portion of the disk. Had to rub and tap pretty hard under water to get it cleaned off.

I had to look up Bryozoans to see what they were. Some in ocean water can be bad to handle.

Bryozoans

John

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