It's some type of algae (possibly brown) - I get it too. I'm not one to be advising on plant-keeping, and can't reply now, but I'll try to do so tomorrow about what I've done that has kept it at bay a little. In the interim, what (if any) fertilisers do you use and what is your lighting regime?
Will have to postpone replying comprehensively until tomorrow, unfortunately, but in interim think: * lighting regime likely the main problem - reduce gradually to 6-8 hours to see if helps; * fertilisers with macro (nitrogen, potassium, phosphorus) and micro nutrients ought to help and possibly addition of CO2, but best/easiest to try the former first.
Following on from yesterday: * lighting regime - best to reduce by ~1 hr per week until down to 6-8 hrs, and see if that helps at all; * fertilisers - macro (NKP) and micro nutrients worth trying initially, and, if that doesn't help, then CO2; * fast-growing plants tend to help combat algae eg hygrophila, floating Indian fern.
In my own case, I had to change my lighting as the ballast 'blew' - new lighting (Aquael LED retro-fit light) has definitely helped. I had been dosing Seachem Flourish and root tabs but not macro fertilisers. However, having very low nitrates, and having high phosphates in tapwater, I started dosing nitrogen and potassium but not the phosphorous; the addition of these macro fertilisers seems to have helped considerably.
I oughtn't to advocate this, as it probably damages the plants, but I do find that either 'lifting' the algae off between two fingers, or even taking out the plants and giving a gentle dab/wipe with a towel or swipe with a fishkeeping toothbrush, helps.