Toto Vespa Water Area Has Turned Yellow

The water area of my Toto Vespa with CEFIONTECT has turned light yellow. I have scrubbed it with Clorox toilet bowl cleaner several times and it will not clean up. Any suggestions that won't damage the CEFIONTECT? What could cause this?

Thanks,

Dave

I know in parts of Issaquah the water leaves a pretty good coating on bowls.
You may want to try CLR
http://www.jelmar.com/CLRbath.htm