It happens... I think the tank and eels look healthy. I have had tanks (usually with mature DSBs and low flow) that had nuisance algae growth like that, with undetectable nitrates. They algae was tying up all the nutrients.
Usually to fight hair algae I'll try some of these:
check whether the bulbs need changing - spectrum shifts with age, and can promote nuisance algae
cut down the light
increase water changes
increase filtration - adding a refugium is especially effective
feed or stock lighter
check make-up water for nitrates
get rid of the sand bed