What do Grad Schools think about Pass/No Pass courses?
I don't think you have anything to worry about. You seem to be doing the right thing in your major and that will be the important consideration for grad school. But anything you do to increase supplementary skills is a positive thing, not negative. For example, even an online no-credit course can increase your skills and would have a small positive effect. If you were doing this in physics courses it would be viewed differently.

You have a good explanation to give for your choices. If asked, just present that. No one should think less of you or your abilities.