Brown hairy algae

could be coincidental. How long has the tank been up and running?

How much do you feed?

What do you feed?

What livestock do you have? I see a Desjardini Sailfin (fav), Blue Hippo, Clown, Bangaii Cardinal...anything else?

What coral do you keep, mainly?

What does your filtration system consist of?

By "levels are just right" what does this mean, specifically, with values (and test kits used to obtain those values)?

What brand and type was the sand/gravel you added?

 

If you have any detectable ammonia in an established tank, I would first suspect the test kit. You shouldn't have any.

Nitrite has no effect in saltwater tanks, it can be 100ppm before it is harmful...so I wouldn't ever bother testing for that

How long has the tank been set up and running?

Have you made any changes, such as flipping rocks, rearranging rocks, or changing flow patterns?

What about Phosphate, do you have a test kit for that? Hanna checker is really the only decent hobbyist grade Phosphate test kit...all others, you can't discern between 0 and 0.25 which is a big range.

Besides the big fish it doesn't sound like you are overstocked and overfed so I would think that you have some built up nutrients in the rock that are coming out. Where did you get the rock from and how old is it? If you know...

 

Phosphate is in all food, nothing you can do about it. Nitrate has a bacterial export mechanism that naturally occurs in reef tanks with live rock. Not so much so with phosphate.

The result is that phosphate in the system can build up, and can deposit into the rock in a couple ways: it can become part of the the periphyton, which is the thin living surface layer, and this is readily 'dissolvable', or 'accessible'...so it can dissolve back into the water after P is lowered, or algae can use it as a source. There is also calcium chemically bonding in a crystalline form with phosphate, and this dissolves back only in pH below 7.0 (death to tank at this pH) or by a special process called bacterial cleaving, which may or may not occur in small tanks (but evidence shows that it may)

Either way, what could be happening here is that you ran a long time without phosphate media and this caused the rocks to essentially soak it up. Eventually it hits a point of saturation where it can't take in anymore, and then you have excess which results in algae growth. So you may have had a long-term low reading of P because it's sinking into the rock. Now that can't happen anymore.

The other thing, which you already mentioned, is the sand. Was it sand or "gravel" that you added? Could also be elevating the P in the tank, if it was not of good quality.