Is it ethical to use already existing material to complement teaching?
If you cite the work you avoid plagiarism, other questions aside.
But the really ethical way to do this is to ask permission first and explain how and why you will use the material. If they have published it online there will probably be no reason to refuse you.
But another issue is how it will be perceived by your own students. Other questions here have revealed that some students feel cheated when this happens. So, make sure that you add value to the slides you borrow and that the value added is clear to your students. You may want to explain the borrowing to your own students as well.
In general, students want to know that you are spending effort on them.