Moodle IS an online management system and at some point you and your students will have to spend some time in front of a computer. But the extent of time spent in front of a computer depends entirely on what you use Moodle for.
Moodle supports delivery of courses ranging from fully online to the occasional use to perform a certain activity or access a resource. The extent of screen time is entirely up to the teacher. The majority of teachers around the world use Moodle in courses that are a mixture between offline and online teaching, learning and assessment. In fact, a hybrid, blended model of courses where Moodle simply supports and/or extends the face-to-face and other activities is by far the most widely used way of using Moodle by teachers and students worldwide.