What is this stuff on my cooking pot?

Question

I just finished washing a pot, and maybe 15 to 20 minutes after waiting for it to dry, I started using it. At the moment I was only boiling water with the top on, but when I came back to the pot, the water had boiled but there was a strange white substance on the sides of the pot above the water. It looked as if someone powdered the sides with flour, however if had no real texture. I tested to see what the substance was by rubbing the side slightly with my finger, but when I brought my finger up to see, nothing was there. Additionally, the part where I put my finger no longer had the substance. If it help to deduce what the substance was, the pot was used to make white rice a few days prior. The substance (to my knowledge) had no taste nor odor. Unfortunately, I did not get any photos as I freaked out a quickly washed the pot (the second time I tried boiling with it, the substance was no longer there).

Answer 1

Scale from the hard water; mostly calcium carbonate. Harmless, but it can build up and clog appliances that boil water like coffee makers.

Answer 2

Your tap water must have a high mineral content.

Answer 3

Tiny macrobiotic particles that  are inside  bacteria and fungus  that reside in your water.

Answer 4

It's the residue from hard water.

Look at the inside of your kettle .. where you don't 'wipe it off' every time.

Answer 5

Either soap scum or hard water residue. 

Answer 6

Use a magic eraser to remove it.

Answer 7

Just minerals in the water nothing to worry about.

Answer 8

Its buildup. Use a magic eraser to remove it.