I have a Toto Drake toilet, and I've noticed a green or dark blue/black area at the bottom interior. It's not mold and won't come off. I contacted Toto's customer service, who suggested it might be hard water/mineral deposits. The rep recommended CLR (calcium, lime, rust remover), letting it sit for 15 minutes, then scrubbing with a nonscratch pad. I used a whole bottle, but it didn't help. Does anyone know what this is? The toilet has Sanagloss, but to my knowledge, no abrasive has ever been used on it--and I'm not sure it would cause this problem even if it had been. Thanks.