One thing I've heard of is changing brands of food! Algae basically feeds off phosphate and nitrate. And some, particularly cheaper foods haves tons of phosphate in them. I've spoken to heaps of people with the same issue and after a couple of weeks of the same water change routine just different food and the issue has completely gone. Worth a try.
The fact that water changes aren't doing anything, indicates to me that it's likely that it could be this, as you remove a volume of phosphate in a water change but feed it straight back in with food!