Never seen anything that bad, but I suspect it's deposit from using the wrong coolant. One of the common coolants found in third party parts stores (Autozone, O'Reillys, etc) has rust inhibitors that are incompatible with the BMW OEM fill, and when used CAN result in corrosion or lime/calcium deposits.
Either way, walk away. Cars that had the incompatible coolant filled over time can develop cooling problem issues that are difficult to resolve, because that deposit/corrosion travels all throughout the cooling system and fouls up the thermostat, water pump, and radiator.
