The echoes are the bony glenwood, the labrum and then a tiny bit of gas (nitrogen) which comes out of solution from the synovial fluid.
You see this commonly,
The differential would be some crystal deposition like CPPD or gout, however this is much less likely.
Yes, gas will move around and not be a constant feature. It is usually a feature of a healthy joint with an intact capsule capable of generating a tight seal and hence negative pressure when moved dynamically.
I see this gas as a sign of joint health rather than disease.