Identification: bright green mould??

I have several large 'plastic planter boxes' on my deck and in getting the soil loosened up and preparing the soil, I noted two of the planters have a 'very bright deep green stain' on the interior walls near the bottom. (I have drilled lots of holes for drainage) but am wondering what would cause the bright green stain?
Should I wash out with bleach and soap or would it be OK to replace some soil prior to planting flowers??
Thanking you in advance
Aplumm
abplummer@shaw.ca