coolant issues

[bolo]

You need to flush. The green stuff is corrosive to the aluminium used in the cooling system.
If the coolant leaks out of the tank when it gets warmed up, it could be a sign that the seals on the lid for the tank ball are stuffed. You should be able to just buy the lid without having to buy the whole tank.

[Transporter]

The green stuff as you call other coolants does not have to be corrosive but in most cases they're NOT compatible with other coolants. You can't mix the coolants you get a chemical reaction and as a result the corrosion in the cooling system.