That is a copper line. Looks like the suction line, coming from the evap. Looks like the evap is either aluminum or coated with something. But that like is fine. The discoloration you see is from using an oxy acetylene torch to braze the lines. Nothing to worry about, it looks in good shape. If they left the door open or cracked for an extended period of time, that will absolutely cause massive buildup of ice and frost. A heat gun or hair dryer is a safe way to melt it, if you need it done faster. If it happens again when you know the door has been closed properly, then suspect an issue with the defrost. But if it doesn't recurr, then you'll be fine. A defrost heater/time cycle cannot handle the massive ice build-up from excess moisture. So leaving the door opened will cause it to freeze up to the point the defrost cycle cannot handle it.