It looks like the beginning stages of black brush algae. Commonly referred to as BBA. There is an imbalance in your tank and it may not be one thing. There are spot treatments with peroxide you can look into, but it's better to fix the tank in the long run. Search the site to fine cures and treatments.
So I’ve been reading about bba. It seems that it’s mainly caused by low or fluctuating co2 or too much water agitation. I’m now asking do I need more fish? I have 4 neons and 4 ember red tetras in a 10 gallon with 1 assassin snail and 2 large and 1 small Java fern and 2 small windelov ferns. The bba affected fern is also directly under the overflow of the filter. The fronds are always slightly waiving. Could this be stressing it? I don’t want the bba to spread to my other plants. Can I cut off all the leaves and keep the rhizome or will it die? I change 35% water every week and test parameters directly before wc. Always ph7.8 ammonia 0 nitrites 0 nitrates 10-20 gh6 kh5 temp 76 with no fluctuations.