First of all, english isn’t my main language and I lack the vocabulary to describe the issue, types of walls/cracks, I’ll attach images instead. Basically the housing complex was built on a cliff like area at least 20 years ago, our house is at the lower part of the cliff and there is this wall(pictures attached)right behind our 1st floor, I’ve noticed huge cracks at least 7-8 months ago. There is also a water tank standing right on the edge of the wall. Is this serious?
Yes, I would be concerned, and have it inspected by an engineer.
BTW, I believe your house is at the lower side of a hill (not a cliff), the wall is an earth retaining wall. The sudden appearance of a long, wide crack indicates the soil behind is moving and putting extra pressure on the wall. The soil movement may be related to the water tank if it is a new installation.