Thank you for all your rapid reply. You guys never found any algae or fungus among the moss? These two weeks are all rainy days in my place. And the wind is very strong. However, yesterday I found there were dark green algae appearing among the moss in my nep pots. The air-movement was very good, since I am growing my neps outdoor (put in a big box, but the box top is completely open). I really hate hate such algae and fungus.