What could green covering on a radiator pipe be?
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.