Have some kind of slime in the hyd system. EX 120-5 Flush it 4 times, changed the filter 3 times. Still not pristine. Noticed some kind of slime in the filter housing. I think there was condensation in the tank since the last two years there were very few hours put on the meter. Found this guy with the same question here: Here Some of the suggestions were on the dubious side. They must have been smoking a doobie at the time.
You'll probably have to use a biocide to eliminate the algae. Perhaps a diesel additive will work. I think I'd call tech support for your oil supplier and talk to an engineer. Here's a link or two to a similar discussion. http://forum.ssca.org/viewtopic.php?f=4&t=10855 http://hydraulicspneumatics.com/200...le/False/6432/TechZone-HydraulicFilter?page=2
"THE MG EXPERIENCE" : the name of that forum in the link says it all! The low hours and your location may have something to do with it. Maybe put a dessicant breather on it (or a new dessicant breather?) and keep an eye on the fluid?
Only the use of a biocide will actually get rid of the algae in the first place. They feed on the hydrocarbons in the oil or fuel and multiply. In the hydrocarbon trade they are known as "hum-bugs" - don't ask me why. Get your local oil/fuel supplier involved. They will be able to recommend something.
What you are calling algae may be additive drop out. I would recommend getting the oil analyzed before putting anything else in your system. It might be that all you need to do is use the machine. Good Luck!