Thank you for the help and tips! I'll check the website.
I think it probably is lime. Yesterday I noticed that some of it had broken away, but there is still more in the bottom and a bit on the bottom sides that needs to be cleaned.
It is a 1 gallon tabletop distiller. I bought it used, from an individual on ebay...about 4-5 years ago. It has been wonderful! No problems with it at all. It's a Pure Water distiller.