In order to really help you we need much more information...
Not trying to be redundant but here goes:
What kind of lighting & how old are the bulbs?
Algae outbreaks are common around the eight (8) month mark on bulbs.
Did you just recently add the sump or has it been there for a long time?
Sometimes with the water volume changes weird things can happen...
How long is your lighting cycle?
Unless you have corals that NEED the light on for 10 hours or more, you can switch your lighting cycle to 6-8 and this could help IN CONJUNCTION with other remedies i.e. water changes, phos-ban...etc.
What kind of water do you use for changes & top-offs?
If you have used tap water in a pinch recently it can contain phosphates and nitrates that will facilitate an algae bloom (among other things).
If you use an RO/DI unit how are the filters looking/how old/do they need replaced?
You said you had a Lawnmower Blenny, has he eaten hair algae in the past?
I have heard some will/do not eat it...maybe they are spoiled...lol
How often and what are you feeding?
Over feeding causes detrius buildup and we ALL know where this goes!!!
I have decreased feeding my fish to once-a-month 4 days before a water change...sounds crazy...but I am AMAZED how much better everything is going and once and a while within that month I will purchase live brine and toss some in. Basically, fish are ALWAYS hungry and they really don't need to be fed all of the time. Just keep an eye on thier bellies and if they look sunken in then toss some tidbits in there, only enough to be eaten!
I also purchased a yellow tang *he was on my 'to-get' list anyway* so he helped in gobbling it up...not a fan of just buying a fish to take care of a problem b/c they have the chance of an early grave when the 'problem' you have is eaten up and there isn't anymore to eat, but timimng worked out on him as we were going to get one anyway.
I had a hair algae problem a few months ago and just by following some of the topics/advice I have listed above I was able to get it under control...i.e. GONE...
Hope all works out!!!