heatingpipe
Recently I have discovered the pipe to my radiator is looking a bit weird. From what I can tell, it does not quite look like corrosion, neither like mold. I think I recall there was some green paint on the pipe, but I am not sure. Neither would I know how it would turn into this. Any ideas what this could be?
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Carefully dig it out (a garden hose may be useful for "hydro-excavating" right next to the pipe, otherwise some careful trowel and shovel work) and figure out where it does run, then you can decide what to do about it. Beware of possible other poorly buried items while you are digging (wires, etc.)
Assuming you have to work with where it comes out of the garage (Point G) and where it goes into the house(Point H), dig deeper and bury it deeper, reconfiguring the line to fit the deeper trench, and insulating it very well (buried hot water line sounds like cash leaving your wallet in a hurry.) Remove the damaged section of line while you are at it, or change everything between Point G and Point H to PEX.
One relatively good technique for insulating underground lines is to build a box of XPS styrofoam around the pipe and fill that box with polyurethane foam; I'm not sure if you'd have to do anything special to make it termite-resistant, those are not an issue where most people I know are using them to protect heating lines running outside between buildings.
If you would prefer NOT to run it where it runs now, that is certainly an option under "and decide what to do about it" especially if you are up for tearing into the house as much as needed for re-routing the line inside the house.
Best Answer
You're seeing corrosion salts forming. In the presence of a water leak, copper will display greenish or bluish crust as the corrosion mineral in the water reacts and dries. Blue-green is copper sulfate, green is copper chloride/oxide. Fix the leak.
Discuss with your HVAC expert to find if you need to demineralize/deacidify the water in your heating system.
This is heated pipe, cold water pipes can show a greenish tinge where a lot of condensation from damp air coming in contact with the pipes condenses and drips off the copper pipe.