I have a Marlo CMCV Combo water softener and there is a black liquid in the salt tank. It is at the top of the water line and only around the perimeter of the tank, about 1/2 inch thick all the way around. Feels like a slimy film, or maybe similar to motor oil. I’ve read some other threads that say mold can form in the tanks. But, I’ve never seen mold that looks like this and sits on top of water, but I’m not very experienced in water softeners. we are on a well, have hard water, and I do have a Res-up automatic feeder with Res Care cleaner. Will attach a photo. Any thoughts on this?