Do You know what type of calla you have ? The coloured callas are usually Zandedeschia although there are white as well. There is also a white one called Aetheopica which is taller. Each variety has different requirements. Anyway, I've seen this with Zandedeschia before; not sure of the cause but I wonder if you could give it more sun? Also for that variety it likes to be moist but not too wet so take care not to overwater. Aetheopica is a semi aquatic so likes lots of water and prefers to be in a shadier position. Hope that helps.

Callas are not true lilies - they are aroids. There is an Aroid forum here on GW that might offer more detailed advice but meanwhile, take a look at this.

Is it possible that you have the cultivar 'Green Goddess'? This is an aethiopica cultivar that produces white flowers flushed with green. Individual flowers are only present for a few days to a week at most.