Grullon has been a decent prospect for years. Not sure much has changed this year though. He still looks to have the offensive limit of a backup catcher paired with good defense. Compared to Alfaro he is much better at blocking the plate. Throwing is probably comparable though not better. Alfaro is a much better athlete which shows in other aspects of the game.
The important offensive numbers for Grullon are the larger sample at Clearwater. .682 OPS after a .695 OPS at Lakewood last year. His walk rate also dropped considerably while his batting average stayed the same. He did show a bit more power but that is likely the park (and he'll show more at Reading like every hitter will).
The promotion looks like a result of catcher injuries and they thought Grullon was able to handle it. It was not really because of performance though and he is not close to Alfaro as a prospect. His Clearwater offense suggests that he probably will hit a wall at some point in Reading when word gets out as to how to pitch him.
This season has been more of the same for Grullon. He is a decent catching prospect who does not look like he will show much offense. Still young enough to improve, but has done little to suggest that he is pushing Alfaro. He has a nice 15 AB to start his Reading career, but it is only 15 AB.
