Hmm, not sure what you're asking - do you mean potting compost? Still in the bag or out of it? Or do you mean your compost heap? And do you mean a green growth, or the compost itself is actually green, even if you pick it up and move it around?

It sounds like there may be algae growing on the compost surface - caused by it being too wet.

too wet or too much shade - you can scrape off the top layer and check whether the soil is very wet - if so, adjust your watering. This assumes the pots you're using have drainage holes in the bottom and the water can drain away freely.

There's probably a dense mat of hyphae on top from fungal growth. 3 out of 30 is not bad. Clean them out to prevent aerial spore contamination to the other pots and start over again.

Fungal growth isn't green though.

No, indeed its not,Stera,it'll be algae, moss or lichen, no doubt.

Algae and fungi often grow together. Prime example, you mentioned it Bambo....Lichen...and they do this in a classic symbiotic relationship which is taught in every biology classroom. Lichen can also be a composite of fungi and cyanobacteria (which also utilizes the photosynthetic process to make food) also known as bluegreen algae. Indeed there are lichen which are a "ménage a trois" which would be a combination of fungus, algae and Cyanobacteria. These are common things found in the crust of the earth and elsewhere. WOW! My long term memory is really kicking in ...Biology 101.

Well done for a good memory Loosestrife! I can't say I learnt that at school, I garnered it at Horticultural College, so your education system seems to be better than ours in that regard. I'm not sure its that relevant to the question asked though ... Fay just wants to get rid of the green 'stuff' on top of the compost and to know the cause rather than identify the culprit.

In fact, its not likely to be lichen, its not been long enough, that usually occurs on plants in pots long term with undisturbed soil, so we're probably looking at algae just on the surface of wet compost.... and a mycelium mat or hyphae on its own wouldn't be green, I'm sure you'd agree...

I was taught about lichen at school bamboo and I notice it has finally returned to the curriculum hurrah!

my first thought was overwatered compost and algae too.

Our 'science' lessons were mostly dedicated to anatomy (human principally, fascinating) and gas therms/units of energy (boring, boring, boring), minor bits of chemistry, that kind of stuff, but then I went to a Secondary Modern - they were big on equipping you for work doing something ordinary (I learnt to type and do shorthand at school) not broadening the mind, lol! Maybe you went to a Grammar...

That's one class that I wished I had taken up and that was typing. To this day I type with two fingers.

Well, it has been tremendously useful over the years, and a source of extra income during winter, when there's no gardening, Loosestrife - to this day, I still type with all fingers, and don't even think about it. BUT, when I was young (when God was a boy, it feels like now) if you were female and could type, that's all you'd be doing for the rest of your working life... Not so now, no one cares about touch typing, I've seen people type almost as fast as me with just two fingers - for short periods, I'll admit, I expect a 50 page document with two fingers would be much slower...

You said it, alot slower. Before I retired I was high tek with one of those type as you speak programs and once it learns your voice well it was fantastic to use. As far as the good ole days, I don't know how they did it all day on a manual typewriter at 90 words per minute and then get up and do it again the next day.

I'd have shot myself in those conditions in preference, frankly - I've always liked typing, there's something very satisfying about the coordinated finger movements, but a typing pool was something, thankfully, I managed to avoid altogether...

Thanks to all of you guys for answering!! The information was very useful. I think it was definitely algae on wet compost, I've removed it and been more attentive when watering now =)