Lynne, try bordeaux spray. AKA bluestone. Available from nurseries etc. Spray 3 times 7 days apart at the mix rate of a level tablespoon to a gallon of water. This will not harm your plants or animals but you should avoid breathing the powder when mixing it. Also, strain it thru an old pantyhose or similar after mixing as it doesn't dissolve too well and will clog up your sprayer.. Also milk is supposed to be a fungicide though i've never tried it. Consider too that fungus grow where there is too much moisture and not enough light. Can you change either or both of these factors? Perhaps by watering by flooding the ground so the rocks don't get wet. Otherwise water in the morning so the area is dry overnight. That's possibly when the fungus will spread.

Do you think this could possibly any other type of disease
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maybe it is Lynne. Can you take a bit of it to a horticulturist or a qualified nursery person and get them to check it out? It may be a fungus or a moss. Fungi are part of natures way of breaking down dead orgnic matter. Which begs the question 'what are they living on to start with".
Try the bordeaux on a patch but definitely get someone to look at a sample i reckon.