You had a carbonate explosion, aka snow storm, which you may not have seen as they can be slow at times. The deposit is that of CaCO3 and MgCO3, basically hard water deposits. This can happen at times if for some reason the CO2 leaves the system quickly. Warm cool water mixing can also cause this. Or really, really hard water with a high Alk and pH. Some times these explosions can't be explained, especially when all water parameters test normal. 99 % of the them are from to high a pH, Ak and Ca due to adding additives to much at a time or wrong buffers.
I would dump the water and start over. It will be along tidiest task with a razor blade. I would suggest getting some Muriatic Acid from a chemical supply store and dilute it 10 parts water to 1 part Muriatic Acid by volume ( as in, 1 cup Muriatic Acid to 10 cups water). Put it in a spray bottle and spray the walls and let it sit for and 1/2hr, rinse and do it again if needed. Be careful with this stuff, it is HCl ( Hydrochloric Acid) where rubber gloves. For starts, try straight Vinegar in a spary bottle first, to see how it works, full strength.