Apples will not cause watery poop. They are given when people and animals have diarrhea because of their super high content in apple pectin which is a soluble fiber than 'gels' so it makes the feces more solid.
If the bird looks depressed and anxious, started plucking more, and has diarrhea, he could be seriously sick so, please, take him to an avian vet asap. This is something you would have needed to do anyway because every new bird needs a complete physical so it's just a matter of making an appointment now instead of in a couple of weeks.
In the meantime, keep him in a calm, quiet (away from daily traffic) and high (at your head or higher level) spot , warm (85 to 95 degrees) and cover the top, back and sides of his cage but not in the kitchen (worst place for a bird).




