Sure...just need to let the carbonation come out of solution so it doesn't cling to the hydrometer and mess up your measurement. Just make sure to not let it evaporate while it decarbs or you will concentrate the sugars.
What is the connection between gravity and apples? That flavor is resultant from acetaldehyde which is an intermediary compound between pyruvate and ethanol. Some yeast strains generate more than others; but, if you let the beer hang out with the yeast long enough it will clear out. If it is present in the bottle, give it time, perhaps you have enough yeast to clear it up.