It probably won't hurt them but it can be unsightly. Is it moss or algae? Either one shouldn't hurt unless it gets real thick, but if it bothers you then cover the surface with some kind of mulch (sand, gravel, newspaper, etc.) that will block the light but will let water and air through. Once the begonia leaves are large enough to block the light to the soil, then that should help tremendously.

I've always wondered that - because I've always gotten a layer of the algea on my soil and people have said it's detrimental to begonias - HOWEVER I've never done anything about it and I've never had a problem... The only bothersome thing about it was when I transplant the plants they turn my fingers green green and it's nearly impossible to get off!! Give a whole new meaning to having a green thumb! :)

I don't know how detrimental algae can be since it seems to grow everywhere and anywhere. Often times when I buy begonias from mail order places I get weeds and algae growing around the base of the plants with little fanfare. I guess if it is okay for FL and CA growers, it should be okay for us.

For tiny seedlings it can be problematic - growing faster than the sowed seeds. It also can give the impression of the soil being wet but many times the soil may be dry right under the surface.

Fertilizer seems to aid in the rapid growth of algae so it can be an exercise in futility in whether to feed the plants or the algae (reminds me of algae bloom in ponds and other bodies of water due to fertilizer runoff). Even starting seed in a sterile mix can have algae growth if the mix is exposed to the air. It is like mold - give it a little bit of moisture and let it go unnoticed and it will spread rapidly.

I know what you mean about green fingers from pots and plants but a little soap and water quickly cleans them up. I guess it could be worse.

Glad to hear it isn't harmful. I did however transplant them into a larger container and they appear very happy.

That's all that matters!