Actually your sponge looks way finer. Open cell pore foam/sponges like the ones in the AquaClear sponges can be found in craft stores, or so I've heard. But the reticulated foam sponges like the ones in the link for the Penguin powerhead are truly the most durable and long lasting, and also able to go a long time between cleanings. Incidentally this is the difference between the Hydro Sponges and the Hydro Sponge Pros.
The problem is not so much that your prefilter will get dirty, but that it will restrict flow so much that it will stop sucking in water, and your motor might burn up from running dry.
I would be writing/calling the Mr. Clean company (probably Procter & Gamble) and asking them about the mildewcides etc. Would only take a quick Google to find out contact info.