I don't like that Hagen Mini E filter because the f-media is too small.
Looks like the white slime is taken over your sponge instead of the B-bacteria.
When you rinse it out, the white slime still dominates a majority of the sponge that why it grows back 1st.
- Are the snails & guppy nibbling on the white stuff?
- What are you feeding the guppy?
- What are you using for the source of ammonia?