"Welcome and introduction" is not a "Lesson" but a "Course". The icon means that that course is open to guests.
Lesson
How to remove icon of face?
གྲོས་བསྟུན་འདི་བསྡམས་ཡོདཔ་ལས་ ལན་གསལ་བརྐྱབ་མི་ཚུགས་
"Welcome and introduction" is not a "Lesson" but a "Course". The icon means that that course is open to guests.