Is my toilet basin erroding or just cosmetic discolouration?

[Skipsteer]

Initially I thought this was a slug in my toilet basin! But after trying to remove it I realised that this is a discoloration on the toilet basin itself.

Any idea what has caused this and if it could cause any leaks and/or other issues?

Also how best to remove it? I tried bleach but made no difference.

Thanks

 

[Wayners]

Lime scale

Going to take a few treatments and several products you can use.
Google it

 

[Hugh Jaleak]

Use some Harpic 100% Limescale remover for WC's. Give a liberal dosing around the pan and under the rim, it'll run down and cover the entire pan, and mix with the water. Ideally need to leave 24hrs, (Overnight at a minimum), and then gently clean with a suitable Toilet Brush, (protect your eyes/face), and you should find it'll dissolve and leave your Toilet lovely and clean.

 

[Skipsteer]

Use some Harpic 100% Limescale remover for WC's. Give a liberal dosing around the pan and under the rim, it'll run down and cover the entire pan, and mix with the water. Ideally need to leave 24hrs, (Overnight at a minimum), and then gently clean with a suitable Toilet Brush, (protect your eyes/face), and you should find it'll dissolve and leave your Toilet lovely and clean.

Thanks very much for your suggestion.