Dear Moss And Stone Gardens,
Thanks so much for your reply. I have looked at it and taken some pictures. It looks like whatever it is, it is totally “consuming” it.
First it turns black and then it just disappears, with a black thin layer of slimey stuff. Is that what black slime mold looks like? And, is there anything I can do. I read about that problem in aquariums, but not in outdoor ponds. Thanks.
The photos certainly do help with the diagnosis. It appears as though you have not only black slime mold affecting your moss but judging from the photos, grey mold as well. Molds are one of the few enemies of mosses and often occur in closed terrariums. Molds are partial to the same conditions that many mosses are. Constant moisture and shade can be a recipe for the slimy and fuzzy stuff to invade. The molds are however temperature dependent and you will likely not find them growing in cooler seasons. I have found that once the thermometer reaches 75 degrees Fahrenheit or above, and the moisture is constant, you should keep an eye out for the attack of the molds. Most of these attacks are easily remedied by removing the moisture, but if the problem is at the edge of your artificial water system, then removing moisture is tricky. You have already taken a good step by removing the moss that is affected.
Trimming or thinning mosses that are growing into the water can also be a preventative. Mosses may reach for and grow into the water but that doesn’t mean that it’s good for them. Our artificial water gardens have a constant water level that natural streams or ponds do not, this consistent water line is not as forgiving as the rising and falling water lines of a natural body of water. When mosses have direct contact with a body of water, they wick the water into the moss mat. This can help feed the mosses growth but in certain situations can also lead to problems. One of these problems can actually be to drain the water system quicker than evaporation does alone, another is soggy soils and slime molds. Eventually these problems will find an equilibrium and take care of themselves, but the results may not be the desired lush moss growth right to the waters edge you envisioned. A little moss maintenance and trimming will help in this case to stop the wicking and soothe the soul.
Best of luck!