Malic acid is the predominant acid in most apples, so the mouthfeel of an apple is similar to the mouthfeel of many wines before MLF. What do you mean by green apple taste, like as in it tastes like real green apples, or green apple candy? What kind of wine is it and how long has it been going through MLF? It's possible that you're actually just starting to perceive a flaw (maybe ethyl acetate, it can have a sweetness to it that could perhaps be perceived as green apple in certain settings). The Grenache I'm making currently has a fairly pronounced aroma of green apple jolly ranchers, but it's been there since about halfway through primary fermentation so I think it's just a flavor specific to this wine. It's possibly just a part of that wine that happened to be un-perceptible until recently. By the way, SG does not change during MLF unless primary is still finishing, so it doesn't give you any indication of MLF progress.