I suspect that if algae are able to overgrow moss, then it is because conditions are marginal for the moss and better for the algae. Healthy moss growing under good conditions will be able to outcompete and even benefit from the algae. Scraping the algae seems counterproductive to me and it would be best to let natural succession run its course. If conditions are right for moss, it will grow.Dane said:The problem that I've had with green algae is that it can cover any moss spores and prevent them from sprouting as well as kill off more mature moss. But scrubbing the would might have that effect as well.