I find it hard to pinpoint an individual element, although I haven't had iron cause thread algae in my established tanks when I tried to raise iron levels. On the other hand though thread algae usually pays a visit to my newly established tanks around the 1-2 month stage. This in both tanks that receive no liquid fertilizers and those that do. Constant removal and water changes has always worked for me, after about a month of thread algae they seem to just go away on their own. One tank however which I completely neglect and have teared down since, always had thread algae problems. I'd remove it every now and then but never looked after this tank as one should, it had too few plants, all slow growing, no ferts, no water changes and too much light.... the perfect recipe for a bad tank
Good luck, keep up with removing it.