From time to time there is some small black sediment coming from a tub faucet in one of my bathrooms. Looks like it could be rubber or plastic. I also just recently changed a toilet in a different bathroom and this was in the water when after I changed the supply valve. Pictures attached. Does anyone know what this may be from and if it's dangerous or not? A local plumber told me it's probably either the cartridge in the tub faucet is deteriorating or it could be from sediment from the water heater. Both the faucet/tub and the water heater are 16 years old... Thank you!