Posted: 10/11/2013 at 12:45
It's almost certainly algae and will be more likely to appear on pots which are in shady areas and never dry out. I actually quite like the effect as it suits my cottage-style garden but I can understand it looking unsightly in other garden styles.
It can be cleaned off by removing the plant from the pot and scrubbing with soapy water. Soaking them in a diluted bleach solution afterwards will stop it coming back so quickly, but it's almost inevitable with unglazed clay pots.