Meh....I get the same (hard to tell from pictures) slimy, grimy film that builds up on Jager heater suction cups. It doesn't happen on sponge filter suction cups or the heater caps. The caps on Jager heaters isn't a soft rubber and don't develop the film. If I remove the suction cups, the only way to clean them well is to scrub with a toothbrush. But what happens next is even more annoying. The rubber starts to harden to the point where they become useless. Personally, I think the issue is with the makeup of the rubber. Doesn't make it any less annoying. You should pay attention to the condition of that rubber cap. If it starts to become hard, it may be a good thing to run the heater with only the glass submerged. Just a thought.
"Why can't my wife see all of this stuff as an investment?"