I've had my current DSB for over 2 years and have watched an interesting range of algae come and go (reds, greens, brown, etc). It's typically just a film on the other side of the glass - doesn't extend into the bed. I agree with not stirring the bed. While it probably won't release any significant toxins from the algae, a well established DSB can contain hydrogen sulfide in large pockets and if you get to stirring around that can cause problems. I'd let it be. Good sign your microfauna are doing their job!