algae

[chewy-tuna]

just done a water change on my system a few months back.
i am using x1 clear blue, same as over a year ago.
i emptied flushed and put new fluid in system.
now after two months i am seeing clear algae build up on roof of res.
res is a dual bay phobya holds approx 1 liter.
the clear blue fluid is perfectly clear no algae present so why am i getting this as a build up on roof of res?

 

[chewy-tuna]

that is what i thought to on x1, so how is it possible to get a transparent alae film build up on roof of res? fill cap is also screwed in tight so no air getting in.
do not know if this is any importance, but the air gap between water level and roof of res is a 1 inch gap which is on safe max level,which is marked on front of res face plate.
forgot to mention that pipe from rad out to res in connection, the water falls from above water level into res, would this be a factor on why this is happening?