Hi Chillybean and Weedwhacker
First of all, thanks for your replies.
Apologies for the image - I reuploaded it! I live in Malta (we have a Mediterranean climate). Sorry you are right about the fuzzy white stuff, I meant to write mold not moss!! Yes I think that that was the reason for the mushrooms etc - infact they stopped growing once we started watering them less frequently. The "terrarium" it is closed up apart from a small opening which is maybe 16 by 5 cms. It has light bulbs so it's a bit hot in it but we shower the plants every day with water. Tbh it is supposedly an aquarium, but for the time being we decided not to use it as one so we thought we would give it a try with plants. We have the area split into two - one side we have cactus while on the other side we have the plants seen in the attached image. Wrt soil we have a layer of substrate and then a layer of plantation soil I believe. The plants aren't dying or anything - they are actually growing really nice - but it's annoying to see this white "soil or dirt" and not knowing what's causing it.
Thanks once again for your help - much appreciated