fleur de sel is much finer and flakier than a typical course grey sea salt. I think the grey salt has much more flavour, however. Frankly, I generally only use fleur de sel for filling salt cellars. If your recipe wants something flakier to go on top (?) you might find the fleur de sel works better, but if it's flavour your after, go grey!

Great -- thank you both so much for the answers! The salt I have is pretty coarse, so I think it should be okay .Was more wondering if there was a noticeable taste difference between fleur de sel and gray sea salt. Doesn't seem like there is. Thanks again!!

Most high-quality salts are probably just fine. It's about getting a good salt flavor to go with the caramel and maintaining a texture intended by the recipe.

I would say yes it should work since fleur de sel is sea salt and is typically grayish in color. I would just pay attention to how fine the grind is between what you have and what the recipe calls for.