I dont understand what it can be??

I use high grade destilled water..
Plates are 316L..
Use NaHo (Mr muscle bluecap pure NaHo)
Hose are black EPDM..
I have condition the cell carefuly @ 3amps and change water many times and refill whit new destilledwater..

I suspect the hose now, becuse the dirt looks like rubberish..

Or something inside my shurflo pump??