I'll bet you have an apple tree inside your Botanical Garden, just outside the view of your screenshot. The floating apple will be gone tomorrow and doesn't seem to affect the game.

I've noticed that when this happens very occasionally I will find an apple in some random place (such as on the beach, or in the middle of the southern marsh). I'll be able to pick that one up. Anyway, I contacted support about this when it happened, so they know about it.