"Green stuff" on bronze coins is not mould, but verdigris - a form of corrosion. A thick layer of verdigris is considered normal, even desirable, on ancient Greek and Roman coins, but on more modern coins it's only bad news, I'm afraid. It's presence on a coin is usually regarded as the "kiss of death", as far as collectors are concerned, unless the coin is a particularly scarce date or type.
There are ways that verdigris can be treated, but it's very difficult to treat the coin in such a away that the coin still looks good afterwards. Ammonia, for instance, is very good at removing verdigris, but it also turns the coin an ugly pinkish-orange colour.
Some forms of verdigris are known as "bronze disease" becasue of the almost biological fashion in which they "spread" if the conditions are right. It can even jump from coin to coin if they're stored all jumbled together in a jar or tray. If it's this contagious kind of verdigris, it's better to treat it in some fashion than to leave it alone, because the coin will become a crumbly green ruin if nothing is done.
If you can, post some pics here on the forum, and we might be able to tell you if your coins are worth trying to save or not.
