The white powder is likely Aldehyde. It's formed by incomplete combustion of the fuel. If it builds up too much, it can begin to burn with a noticeable odor. Many people mistake the smell for a "new smell" or material within the enclosure burning off... it's not. There are 2 things that can reduce the buildup: reduce the fuel or increase the air in the combustion process. Neither of these are in your wheelhouse as a consumer or service tech. These systems are atmospheric in nature, meaning they simply introduce fuel into a natural atmosphere where combustion occurs. This is a very typical problem with atmospheric combustion including standing pilots. My gas fireplace also creates this white powder. We enjoy the yellow flames lapping at the logs (and that's the problem). A clean burning fireplace would not be very appealing to the eye, think blow torch or furnace power burner. Simply clean the powder from the effected areas regularly. A knowledgeable gas provider will give you the same advice. Also... we aware that when incomplete combustion occurs, NOX related gases are elevated (CO2, CO). Keep your family protected with quality CO detectors.
