Bronze Disease On Straits Settlements 1845 Halve Cents?

@coin, not sure that it is, but if since you suspect it, you definitely need to quarantine this coin from your others.

Is the green adherent or powdery?

If the corrosion is not powdery it means it's stable and not active. If powdery then verdigris is active and can destroy the coin, it needs some conservation. There's a big difference between conserving a coin and cleaning it. Do some research about techniques to conserve a coin. For example some lightly dip silver coins in acetone that does not damage the surface. Most collectors say DO NOT clean a coin, especially with anything abrasive.

There is a product called Verdi Care some have used, including me that won't clean the coin but will help it's appearance and help stop further corrosion.

In the search line here on CCF, type in Verdi Care and "coin conservation" "copper cleaning" etc. and you'll see posts about this.

Some have used a method of soaking an ancient coin in water then baking it in an oven to get rid of verdigris. But I repeat, do your research, learn about various methods before trying something like this! You could ruin the value of a coin if you mess up. There are pros who conserve coins, such as at PCGS and NGC, but I don't know how much they charge, would be worth it for a high value coin.

Come out from the coin.