Grease almost like GEL INK

Lately I have been seeing black substance almost like grease, or disolved rubber. I have only seen it in the tubs. Since we mostly was in cold water. This stuff stains the tub very easy when you press on it to get it up. It acts a lot like GEL ink. I put in a new HOT WATER HEAT about 1 year ago. What is this and what do I need to do?
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