hookedonreefs
I run a 75 gallon tank too, and I think you need more water circulation. The tankwater looks stagnent to me.
A Mag7 in the sump, and (3) Rio600's may not be giving you enough water circulation - especially down deeper in the tank.
Here's what I would do ....
1) I'd stop feeding the clean up crew. They'll get lazy - keep them hungry.
2) Run activated carbon for several days - change it - run some more.
3) After I clean it - I would run that skimmer full tilt 24/7
4) I would test for phosphate for sure.
5) Make sure your alk and pH are up where they should be.
Low pH encourages nuisance algae growth.
6) Move the water more - get it moving from bottom of tank to top.