Looks like a sintered bronze muffler, sort of the same thing they used in older carburated auto in-line fuel filters (if they didn't use the pleated fiber ones).

I'd unscrew it and screw in a real filter. IIRC 3/8" or 1/2" pipe is the thread size.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).