Most algae is the result of an improper balance between lights ferts and Co2(or Excel).
Looking at this picture I'd say lack of maintenance which caused high levels of nutrients in the water. For example...what is that behind
the leaves/w the hair algae on them ? Behind and to the left that is. And almost directly underneath the plant/w the hair algae...is that
a decaying leaf ?
Hair algae will also cover the leaves of plants that don't grow fast(like a Crypt) when you have lots of light in there.
I have two ten G tanks. One has a small amount of BGA in it. This tank has two T8 bulbs on it and very little algae due mostly to
a fairly good balance of light/ferts/Excel. In general two bulbs is too much for a tenG. But not/w those other things added.
The other of my tenG tanks has T5 lights but does have cultivated algae in it on purpose.
Trying to answer with as little detail as is available to me.
Specs would help. Light including type of bulb(s)...ferts if any...Excel ?
P.M.'s are answered. Try to use this site to get suggestions on water changes.
Try to remove any dead plant matter and even trim obviously dead leaves from live plants. Then do a water change as per suggestion
on that site but follow it by another one after two days. Then use the suggested schedule from then on. This will help to remove
some of the excess nutrients from there which the algae is feeding on. No direct sunlight but only when viewing the tank.
A timer is best, and Walmart has them for $7. You should not exceed 8 hrs a day till the algae is gone. If you do these things it
will die off slowly. If you only have one bulb and no direct sunlight, a Zoo Med Tropical sun T8 will help as it's a toned down light that will
still grow plants but may not be necessary. Try the other things for three weeks first. Use no ferts for a while till this is gone.