Formula turned green

I’ve noticed that the formula spots in the bottle or if there is a very small amount left at the bottom of the bottle it will turn green after a while. It usually takes a good amount of time to turn (late night feeding leave the bottle by the sink and notice the color change) but today my LO drank her bottle in 15 minutes and there was a little left in the nipple when I took it out I noticed it had turned green. Does anyone know if this means the formula is spoiled? Its a bran new tub I opened, but I’ve never seen the milk turn green this fast, it usually turns after about 45 minutes almost an hour. Baby is fine she’s not crying or throwing up yet, but Im keeping an eye on her. Please let me know if this has happened to you or If you have any information, I can’t find anything online, everything I find is about green poop.