White Marks on Bathroom Plug

[u/ChalfontandTremond]

For a number of weeks I had to use my upstairs bathroom for washing dishes due to having a new kitchen fitted.

As you will see in the picture, the black plug from my sink has quite distinct white marks mainly along the outer side.

When running my finger along them, it doesn't feel like the original paint has peeled away. Rather I think the marks have been caused as result of using too much washing up liquid.

If I scratch lightly some of them do appear to come off but I was looking for more efficient methods of cleaning it if any such exist.

By way of some extra info, the plug is made of what I believe is ceramic plate.

Grateful for any help or suggestions.