I always thought of the efficiency of natural selection in the context of molecular evolution. I.e. that linked selection and smaller population size cause less efficient selection. It took me a while before I even noticed that people also say the efficacy of selection.
When researching the answer myself, I found that older papers talk more often about efficacy (like this one). I also found that besides the natural-selection context, it seems efficiency of selection is also used in the context of breeding (like here). However, you can certainly find cases when used in the context of natural selection. Finally, there are about 4x more papers that contain the words "efficiency" and "natural selection" compared to "efficacy" and "natural selection".
So, is effective and efficient natural selection the same thing? Is "efficacy of selection" and "efficiency of selection" be used interchangeably?
-- EDIT --
I am interested in the use of these two words in the context of natural selection only. Unlike in other contexts, in natural selection dictionary definitions of the two words do not make much sense.