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It is a food to many animals! But this looks like an infection. These are drought tolerant plants, and will rot in soils that remain moist, at all. This is, likely, what is going on. I would not supplement watering, at all, if you are watering them.

You can try a fungicide, but most microbes cannot survive in the dry conditions that these plants require. Usually, providing dry conditions will prevent any infections from entering, or taking hold in the soil.

Here is an article that will offer more information on their care:

https://www.gardeningknowhow.com/ornamental/flowers/rhoeo/growing-rhoeo-plants-in-the-garden.htm