Corrosive/ Acid cabinet gets wet from time to time
The white powdery crystals are probably $\ce{NH4Cl}$, that is formed thanks to leakage form amonia. You can try to measure the pH of the wet layer, should be acidic because of the leakgae from other acids bottles that condensate on inner walls. Most experiment results should be ok, but if you are working in an analytical lab, the results can be shifted.