Blanket Weed algae. You may or may not have suffered from this algae before - it's certainly not the worst but it look like below:
How is it caused:
Low nutrients, low CO2 levels and poor water movement.
How to get rid:
Very simple - increase your fertiliser dosage and increase CO2 levels (use a drop checker like this one). Make sure your water distribution is really good - I know I say this every week but that's because it's very important. If your plants aren't swaying in the current, then you can bet that your nutrients and CO2 are certainly not getting to those locations (which will result in algae). Getting the basics right makes everything flow as it should.
You can physically remove this algae should you wish. Use your tweesers or fingers if need be and just hoick it out. If you see this algae coming back for whatever reason, just tweak what you're doing and in most cases simple add a bit more of the usual (nutrients and CO2).