Mine has accumulated some algae. I do my best to remove this from time to time but the magic of the Velcro like surface makes it amazingly hard to clean anything out of it. It hasn't ever had enough on it to be a problem. The best way I found to clean mine is to scrub it on a towel or something equally as bristly.
The fun of it is perfecting the cross corner motion. Just be careful, the first time I accidentally released the inside from the outside I was amazed to see how quickly it shot across the tank to meet back up with its other half. Nothing like having a magnetic projectile in your tank : )
With a little practice I am able to do all 4 panes of glass without getting my hands wet.