Candi Sugar in a Pale Ale

Do you mean rock sugar? If so, replacing some base malt with some rock sugar will result in a drier beer. You may or may not like that in an APA. (I wouldn't.) It won't add any flavor because it's just simple sugar and it will all ferment.

Or do you mean candi syrup? If so, what kind?

 

From a brewing perspective, clear candi syrups are basically equivalent to rock sugar, but the darker syrups can be useful. Candi syrups come in a variety of colors, depending on how much they were boiled. The boiling develops melanoidins, which provide flavor. Think about crystal malts. Candi syrup is a similar idea, but produced by boiling sugars instead of by roasting the converted sugars in crystal malts. In both cases, more/longer heat yields stronger flavors. If you look at the flavors from various colors of crystal malts (vaguely sweet ---> caramel ---> toffee --> rasiny --> pruney...), candi syrups have a similar progression. I will add though that I have tasted a stone fruit flavor from candi syrup (D-180 specifically) that I've never tasted from crystal malt. (I'm sure there are probably other unique flavors too.)

 

Well, technically, prunes start out as a stone fruit.