We recently completed a remodel that included replacing a standard water heater with tankless model. Post the remodel we noticed a black, almost rubbery substance "growing" in one of the sink drains. There are two separate sinks in the bathroom and only one with this issue. The first time I took the p trap out because the sink wasn't draining well and the black stuff had formed almost a seal across the drain pipe. We washed all affected areas thoroughly with bleach and reassembled but it has come back. For reference we live in Dallas, TX. Anyone have any idea what this could be?