Not much of a worry if in more sun than shade. And even where a rock mulch is in shade in my garden, they don't encourage moss or algae growth. Concrete sure does though! There are organic products you can use to help control if it does become a problem - Safer makes a moss and algae killer, as does Bayer BioAdvanced (both potassium salts).

Personally, I'd be more concerned about weed development. Rock mulches are notoriously hard to keep debris free and weeds will sprout freely (especially in our climate!). Not as easily removed from rock as they are from an organic mulch.