What is this happening on geraniums ?

I think that's rust on the leaves, so a fungal disease.

G&R

Outdoors, it normally follows a wet spell.   

I'd clear up and bin any fallen leaves, and remove the worst affected leaves. Then spray the plant with a fungus spray that's suitable for ornamentals. It's probably a good idea to spray any other geraniums you have too, even if they aren't showing signs yet.

G&R