So I brewed a mild with some smoked malt.OG if I remember correctly was about 1.044. It sat in primary for about 6+ weeks.
It sounds like your yeasties stopped fermenting early, so all that extra time in the fermenter was wasted.
Some questions for you:
What yeast strain were you using? Some yeast strains produce more fermentation byproducts than others. Also, some strains are notable for flocculating before they're finished fermenting.
Did you make a yeast starter? Low yeast cell count can stress the yeast, meaning they make more acetaldehyde during the growth phase of their life cycle.
What temperature did you ferment at? Low temperatures for a particular yeast strain can also make them produce more acetaldehyde.
Did you suffer any temperature swings during the first couple of weeks for fermentation? Temperature swings can shock the yeast and make them flocc prematurely.
What percentage of adjunct sugars did you use? Anything much above 20% adjuncts doesn't give the yeast sufficient nutrient, so they produce more off-flavors.
Did you fully aerate your wort? Again, lack of oxygen in the wort can strain the yeast making them produce off-flavors.
So, I have read that racking off the primary too soon may cause this, and that aging may help.
Both of these statements are true, but they're unlikely to apply in your case. Under normal conditions, a "best mild" like you've brewed should be fully fermented in about a week and should be fully conditioned after 1-4 weeks. So, 6 weeks in primary should have been more than enough time.
Aging won't help much if the yeast has already flocculated, unless you age for a long time and let the acetaldehyde break down or oxidize into other compounds.
Does anyone have some specific experience with it? It sucks because I was hoping to share it next weekend...
I've had and seen a few batches of green apple beer. In terms of aggressiveness/danger, here are some things you can do:
1) Don't mess with it. Your yeast might have time to reduce the acetaldehyde by next week. If the green apple character is still there, live with it, or make cider from apple cores and serve it with a garnish of apple and a dash of apply syrup/concentrate.
2) Warm the beer to 65-80 F if you've been storing it cold. This might get the yeast back to work but risks speeding up oxidation of your beer.
3) If you've got any trub/yeast slurry at all in the bottom of your keg, carefully stir it back into solution. Optionally, combine this with option #2.
4) Carefully stir in 1-2 cups of fermenting wort from another batch and hope that the actively fermenting yeasts reduce the acetaldehyde without contributing off-flavors (diacetyl, more acetaldehyde).
5) Aerate your beer and hope that gets the yeasts to work.
If you go with strategies 2 or 5, drink the beer soon, since you've set the clock ticking on some really nasty oxidized character. Of course, since it's a mild, it's not going to age well at all, so you'll have to drink it quickly anyhow. Personally, I'd go with strategy #1.