Even slightly damaged seal can effect price of the panda coin-seal as well as coin is a part of Chinese Mint issue.
Don't remove the mint OPM. Besides losing value, I have seen red spots on Pandas even with the original seal.
NGC offer a cleaning/restoration service but don’t try to clean a Panda coin yourself. I have recently noticed a red dot on a graded Panda I have, it’s been stored well with many others for years which are all perfect.
Congratulations on your new Panda coins. Best wishes, Peter Anthony NGC Consultant China Pricepedia — Chinese Coin Prices and More The Gold & Silver Panda Coin Buyer’s Guide 3 www.pandacollector.com