Thanks once again for your help. We have infact reduced the frequency of watering the plants (used to water the foliage plants every coupe of days and the cactus once per week) but now we are first checking (manually) if the soil is still wet before watering the foliage plants. I tried taking a clearer picture but unfortunately it's not easy cause to take a clearer photo, i would need to take the lid off and if I take the lid off, there is not enough light in the hallway (even with lights switched on) for the white stuff to show properly on the photo. I will take your advice Thanks a lot all of you for your help!!
Thanks a lot all of you for your help!! 
