I'm sorry for you to have encountered such a situation.
The only thing we can do about this is to add a quick note on our product's website saying that the moisture sensor is not corrosion resistant.
It's quite usual to see more emphasize put on the product's qualities than on it's shortcomings.
What I can think of for now is for you to buy a corrosion resistant sensor like this, desolder the existing connector and then try to replace it with a Grove connector. I'm not sure how the pins' alignment would go, but this would be the first thought I'd have.