Yes, that yellow line is where the pump draws in air. where it connects up top is a muffler of sorts to quiet the air intake. If you do decide to bring in outside air, I would recommend using a much larger tube like flexible 3/8" silicon, vinyl or even pex tubing to minimize restriction. You can just forgo the air silencer because a long line of tubing does a much better job and wont restrict airflow. moving the air intake outside cut my skimmers noise in half.
However, that air line is tiny and the pump likely pulls an equally tiny amount of air so I wouldn't expect a big ph jump. unless your air pump is drawing outside air, the air stones will just negate any difference the skimmer makes. Besides air stones dont really belong in a saltwater tank, they create a lot of salt creep. Lastly if you don't have tight fitting glass tops you will be fighting a losing battle.