Hi, Kate,
Loved the plants
we traded a while back under a different handle elsewhere.
I, too, have lots of moss in my semps. So far, it has not been a problem for me, other than some anxiety over thinking the plants are too wet. I've never had to treat moss but understand that iron sulfate (ferrous sulfate) is the most recommended way of doing what you want. I could not find any references to the treatment being harmful nor safe for sempervivum and relatives.
Moss is not known as being highly competitive with other plants. The general advice is that if you are experiencing a lot of moss, then the area where the moss is growing is out of balance for some reason. Too much moisture, too much "trash" in the ground (read:nutrients) getting too much light, not enough actively growing plants are the main causes. If plants are planted closer together, then you will find that you do not have a moss issue with them. I know that in those cases where my semps are over-potted (pot too big for the plants) I will see moss. So, if your plants are in beds, try planting them closer together. With semps, that may mean your rosettes do not get as large as they might otherwise, but the moss should abate. If in pots, try keeping them in smaller pots until they really need a larger size.
You indicated you wanted to try a chemical solution, but we do not know if it will be safe for the plants. In cases like this, probably you will need to run your own experiment. Choose some plants for which you have duplicates or plenty, choose an area that is isolated so that run off will not get other plants, then treat with recommended dosage of your moss-deterrent of choice and watch for the results.
When you get your results, please share them with us as I suspect your problem is more common than one might think.
