The pics in post #26 look very similiar to something I've been battling in my tank for a while now. I've assumed it is Bryopsis due to the feathered appearance.
I've tried increased water changes, increased replacement of GFO, manual removal, lettuce sea slugs, all to no avail.
In February, I decided to try removing some (most) of the rocks and dipping them in a 1:1 ration of saltwater and 3% peroxide. I then scrubbed the rocks with a toothbrush, rinsed them and placed them back in the tank. It wasn't much after that, that this stuff started coming back.
On March 6, I received 1 gallon of Kent Tech M and began dosing to increase my Mg levels 100 ppm per day until I reached 1800. Within a week of beginning this dosing, I saw a LOT of die off of this algae, which also furthered my belief that this is Bryopsis.
Now, three weeks later, I still have this stuff in my tank. My tank is much more clear of the algae than it used to be, but what is left certainly isn't going away. The tips are clear, but it's not dying off like it initially did. Basically, what's left is hanging in there.
I plan to keep dosing the Tech M and hope that longer exposure will eventually kill of the remaining algae.