There was a recent thread about algae on the alt.auto.mercedes newsgroup. Check it through www.Deja.com
if you like.
Couple of things that came up: if you want to clean it be prepared to spend a lot of effort doing it. One of the options was removing the tank and boiling it... Another problem is that using an algae fighting product will produce a lot of dead algae, which may do more harm than good by clogging lines and filters.
On my car I'll peer in the tank from the access thing at the top when I clean the sensor and unless I find creatures lurking in there I'll leave it alone.