What is the white powdery substance on the metal panels.

The white powder is known as white rust.

What you are seeing is classified as white rust. White rust can occur if a stack of galvanized metal gets wet from rain or condensation. Water can enter through the sides and ends of tightly stacked galvanized metal via capillary action, (similar to wicking) and cause white rust. The white rust is a depletion of the zinc which is the galvanizing of carbon steel. Light white rust represents minimal loss of galvanizing/minimal loss of service life. Heavy white rust represents significant loss of galvanizing/significant loss of service life.