That's right, we have already had this topic here before. So far, nothing is visible yet, but I have to say that it has been rather rarely wet here in the last few months, only heat heat.
If algae, then spray them with Schuroco Moos weg. What works on gravestones also works on pebbles. I love this stuff; it even keeps teak decks on ships moss-free. With long-lasting effect. Karsten
Here is our version with granite. Shortly after the rain. Moss should be removable with a pressure washer if the stones are large enough. I just noticed that the gravel has settled a bit. I need to refill that.