Thanks. Next time I see it I'll swim down with a dive mask on and have a closer look, instead of just vacuuming it up.
I did just finish a couple of hours of removing the sand (thank you, Shop VAC) and checking the laterals and they all looked good. I have two labs, and the amount of dog hair in the sand that hasn't been coming out when backwashing was incredible. Luckily, it was all clumped up and easy to remove. A lot of the sand seemed very compressed as well, and it's now all loosened up.
After putting the sand back in, I backwashed and the water in the sight glass was really dirty. Same with the rinse. I guess 5 years of crud embedded in the sand wasn't being removed in a normal backwash - not until I loosened all the sand did it wash out.
Anyhow, I'll check what I think is sand to make sure it's not dead algae the next time I see it.