woh, this is concrete cancer and can see the reo rusted stain (reinforced steel) on the conrete. When one such piece can fall apart, that means the rust could spread throughout the whole concrete slab. When metal rusted it expands and cause concrete to crack and fall apart.
This is a very serious problem in building defect and reparing is expensive and there maybe more such defects in the block when one cancer was found. Hope the concrete cancer does not spread to the reo of the load bearing walls or columns. Better ask for SER and move out and ask where is PAP good track records? Using cheap unskilled concrete labours or cheap materials?