Yes, it looks like a cyano outbreak to me. Using a product like Ultra Life Red Slime Remover is generally a mistake. While it can work, if you don't solve the underlying problem all you will do is kill off one nuisance algae and it will be replaced with another nuisance algae.
Statements like "Parameters are fine." do not convey any information. We don't know what you tested or what your readings are. What you think is ok might indicate a problem for someone to.
You system might possibly be overstocked. An RSM 250 only holds about 65 gal of water and this gets reduced when you add live rock and a sand bed. I noticed two hipo tangs in there. Tangs are big dirty fish, and likely contributing to your problem. I'm not trying to play "tang police" but this fish should have about a 150 gal tank for one of them. Consider returning them to you LFS.
While we are on problems, I also saw that you has some asterina starfish. These are generally considered a nuisance creature and should be removed.
I have posted this before, but it is applicable to your problem. The main one I would suspect is over feeding, but check everything else too.
Standard Lecture #2 - Algae Control
Algae control comes down to controlling nitrates and phosphates. If you have a problem with algae it is because these two nutrients are out of control. Do not think that just because your test kits read zero or low values that you do not have a problem. In many cases the algae is removing the nutrients and growing. This is why there is a problem.
Here are possible sources of nitrates and phosphates -
Feeding, especially flake food and not rinsing frozen foods before feeding.
Using tap water to mix salt. Always use RO/DI water for this.
"Dirt traps" and "nitrate factories" in the system.
Low quality carbon can leach nutrients.
Low quality salt can sometimes add nutrients. This is unusual today.
Livestock load on the system
Here are possible ways to remove nitrates and phosphates -
Water changes. Change 1/2 the water and you reduce the nutrients by 1/2.
Skimming. Remove the waste products before the biological filtration need to break then down.
Nitrate and phosphate removal products.
Deep sand beds.
Refugiums.
Algae Scrubbers.
Each of these has advantages and disadvantages. Most people that control algae well use many of the above methods.
There are also other items that can effect algae growth rates.
Good clean up crew.
Other livestock that eats algae.
Low general water quality, especially when the readings are off.
Lighting, sometimes you can reduce it, especially in FO or FOWLR systems.
Old light bulbs. Colors change as they age and this can be a factor.
Water flow. More flow will often help keep algae down.
Manual removal. Very important, especially when there is a big problem.
