It's hard to tell from just that picture... could you describe the problem as well, to give us a little context? Things to note are whether the affected leaves are at the bottom or the top of the plant, whether there are patterns to the color change (like green veins with yellow between), if there are dead spots (light or dark colored), and how the rest of the plant looks in general.

Green stuff is likely algae on the top of the soil, which definitely suggests the soil is too moist for too long. I trust you have drainage holes in the bag, but they may be clogged or inadequate. Put your finger into the soil up to the third knuckle: you want the soil at that depth to be moist like a wrung-out sponge, not wet.

Overwatering can cause changes in the leaf color, definitely, but I can't tell you for sure yet what the leaves are indicating. More information would help a great deal. :)

--Alison

Hi,

Thanks for your reply. Plants look healthy in general and are growing to a good size - also some are beginning to flower

Small lower leaves are turning a lighter green/yellow from the outside in (outer edge inwards) However some towards the middle/top are doing the same.

One or two towards the lower of the plant have a couple of bleached spots on them - but only a couple

Does this sound like anything you know?

again, thanks so much for your help
sam

Combined with the algae on the soil, I would stick with the theory that the leaves are yellowing from too much water... it can deprive the roots of oxygen and cause poor root growth, making it difficult to take up enough nutrients (chiefly nitrogen). Check that moisture. If you can get the soil moist but not wet, then feed them lightly, that should set them straight.

Hope that helps!

--Alison

I really appreciate your help.

I've reduced watering to every two days - but touch top of soil and if still damp I don't water. I've also added some more drainage holes to be safe.

Will see how it goes - plants are growing very tall and are flowering. Fingers crossed.

Thanks again

sam