Moldy cup?

[excyan]

After last posting here a few periods have come and gone and I can happily say I made it work! Finally! Apart from residual slobber the cup and I have a stable relationship

Comments

[m03m]

Looks like limescale to me. Can you rub it off with a cloth soaked in vinegar?

[enleve]

If it is residue from hard water, then soaking it in some sort of acid would help. That could be white vinegar, citric acid, or squeezing lemon juice, or whatever you have on hand.