Trying to determine all that's involved in replacing existing cracked cast iron main drain with PVC. Found the crack causing the leak on the 1st floor just below the kitchen ceiling. I noticed something odd (lack of expertise) branching off the cast iron pipe over the kitchen ceiling. And I'm concerned it too needs to be replaced because the kitchen ceiling has water stains. My original post was going to be to confirm steps of replacing cast iron main drain three floors 2 bathrooms with PVC but now my question is what is this? Is this grayish stuff a previously patched up leaky pipe going to 1st floor bathroom or a specific fitting of some kind?