This is a really common observation.
The thing to remember is that the anterior band of the UCL in the medial elbow is part of the elbow joint capsule.
This appearance is synovial fluid in the elbow joint beneath the capsule (UCL) and the echogenic material in the fluid is fat in the joint which is almost always present on this image. You see fat beneath the ligament if there is fluid or not.
I think this amount of fluid is physiological, but I would always check the coronoid recess and the annular recess for fluid to establish that they don’t have a pathological joint effusion.