I made my cider in September. It always seems to get more and more alcoholic over the months. So last month I added 2 quarts of plain cider to “cut” the alcohol. But now it’s tastiing musty. Any idea why? I leave the cider in a plastic container with the air lock attached.
Any chance that the juice came from overly ripe apples?
What type of plastic container are you using?
What yeast did you use? If you added fresh juice into a finished cider you may have started the yeast up again and that is what you’re tasting. I made cider with bread yeast and for the first month it tasted like cardboard/musty, but after a while it really cleaned up and was excellent. If it is done fermenting you may want to bottle it.