Since nobody said this...
It might be flux. Corrosion doesn't come off with alcohol, but flux comes off extremely easily with 99% iso alcohol. Granted, flux can corrode over time since it's normally used as a cleaning agent to remove oxidation (meaning that it could go through metal given enough time), but I've never seen it happen. Chinese boards aren't always cleaned 100% clean of flux, so chances are that some splashed on there and it just kinda stuck. The longer it's left though, the harder it is to clean. Try scraping at it with a toothpick and then attempt to clean it again with alcohol.