I noticed a white, slimy substance starting to collect on the rubbery parts of my heater. I don't see the substance anywhere else. Do you have any idea what it might be? The heater is a pet store brand's model, and the top and bottom of the glass tube have rubber "caps" with horizontal ribs in them. It looks the same as the top part of an Aqueon heater (not sure if it's the same material). I'm considering exchanging it for an Aqueon, but am not sure if this happens with those too. I called the store, and this is the first they've heard of it.