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Someone else that is more versed in apple disease can help, but just a follow up question to get more information. Is it isolated to this spot only? Or is it covering the entire tree. New growth only? Northern hemisphere I presume? I know chlorosis can mean a few different things, typically starting with over/underwatering, or an iron deficiency.
Sorry I can’t help more, but with more information I’m sure someone can point you in the right direction. Good luck 
Check the underside of the leaves for sucking insects, etc. Looks like it could be aphids to me, in which case a strong spray of water may discourage them enough, or you can use insecticidal soap. Aphids are encouraged by ants; a sticky barrier around the bottom of the tree trunk can help.
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What apple variety is this?
It could also be a virus. There is a leaf in the background with yellow specs on it and that is a strong sign of virus. I have virus in a few trees and it doesn’t do a lot of damage overall. Don’t propagate that variety though.