Mike is correct that unlike the Hour of Code activities the green bubble means something different in CS Discoveries and CS Principles. Generally all the green bubble means is finish. In CS Principles there are some levels that have certain checks going on in the background but even those don't always guarantee that the work is correct. It may just check that they used a certain type of block.
Since CS Discoveries is still under development we have not added any validation to the levels. Many of the pilot teachers have instead found success with telling their students that they would be randomly picking certain levels from each lesson to check and grade. This requires checking the students work on a couple of levels but since CS Discoveries is highly creative we think this may continue to be necessary for many levels indefinitely.
Hope that helps!