First thing, check your phosphates, secondly, make sure there is no stray light coming in from any direction. Thirdly, if your bulbs are getting old, they may have changed spectrum and may be the cause of the hair algae. Fourthly, snails wont eat hair algae, you'd be best of getting a sea hare (However, don't just buy it for a small bit of hair algae like you mentioned). IN conclusion, if there is a single source spot, I would guess that there is some sort of stray-lighting hitting that area causing the growth. Hair algae usually affects us all at one point or another, I wouldn't get too concerned about it, but run through the possibilities and you'll figure it out
