Why does my duck egg look like this?

I have 3 ducks, two we think are English Aylesburys and one Indian Runner Duck. I think (?) that this egg is our Indian Runner but I'm not 100% on that.

Today I went down and only got two eggs but this evening I found this egg. I'm not entirely sure what's happened but the crust layer seems to actually be part of the egg itself rather than being pecked.
Does anyone know what's up with it and if I should be concerned. All the others have been fine for months, and the other two I got today are completely normal.

That looks like a calcium deposit:

This can occur if the shell gland is defective or if there are disturbances during the calcification process during egg formation and deposits can be observed on eggs of any color. Too much calcium and/or vitamin D in the diet also can cause calcium deposits on eggs.

https://extension.uga.edu/publications/detail.html?number=C1255

It will be interesting to see what others suggest.

Hi Thea,
Welcome to Permies.com

My first thought was also calcium spots, but that does not look like calcium spots that I have run into before. I was not sure from your description, but one of the pictures I saw looked like the spot was indented? Like below where the shell would be normally? Or is that just the way the picture looks? All the calcium spots I have run into were on the outside in addition to the regular shell.

The link that I included above gives examples of several abnormalities.

Thea's egg could be a combination of a soft-shelled egg with some calcium deposits.

A soft–shelled egg has a thin layer of calcium deposited onto the shell membrane and the egg has a paper–like feel. If a soft–shelled egg is held, it caves to the touch. Soft–shelled eggs are more likely to be observed in older hens. Other potential causes of soft–shelled eggs include too much phosphorus in the diet, moldy feed, salty water, or feed contaminated with mycotoxins. A soft–shelled egg cannot be processed because of its poor eggshell quality.