Water turned green in colour in a water well

Hello team,

My name is Pradeep from Pattambi (Kerala) we have a water well and it's been almost three years we are using the water happily. However, from past few weeks we have started noticing that due to summer the water level went down and we can see green colour layer in the water which has now turned the water green. I would like to know reaons and how can I get rid of this and also whether it is healthy to drink this water

Thanks
Pradeep. M