Filaments of brown algae : problem in established tank

I have a serious problem in my 55g reef. This tank is about 5 years
old, and is well established. I've kept reefs for about 15 years, so
thought I'd seen everything, but this has me beaten.

The symptoms are that everything is covered in brown algae, which is
more dense nearer the light (10K MH 250W). After the lights have been
off for most of the day the density of the algae is reduced. As soon as
the lights come on I can see filaments of brown algae, like hairs, an
inch or so long, floating around in the current. After the light has
been on for a while, the rocks and corals seem to get covered more and
more by this stuff.

So I have been reducing the lighting period gradually down from the
normal 9 hours to 1 hour.

My corals don't like it: I have a ton of hammerhead, all retracted.
I've already lost a clam.

Fish are a Picasso trigger, hawkfish and a striped damsel. There's a
brittlestar in there too. The fish seem to be quite happy, but not the
corals.

I have activated charcoal, some Kent phosphate/silicate remover, a
skimmer which is working well, and a couple of powerheads. Water temp
is 78F.

What should I do to investigate further and eliminate this problem? Any
advice?

Thanks,
Julian