Not saying what you have is cyano, b/c I can't tell but...
I have cyano in my 40 and got rid of it with maracyn but some it stuck around in the sand, it kinda comes and goes but as long as it stays in the sand and doesn't spread to the rest of the tank I'm not worried and even if it did start to spread I would just dose maracyn again.
A little algae under the edge of the substrate is pretty common. Consider that there's still light there, but not really any flow. In my experience cyano doesn't just take over tanks unless you're nitrate starved. Every time I've had it I've added some nitrate, removed the loose stuff, and had it be gone within a week. I'd guess the stuff on the glass to be something else based on the location.
You can bury the stuff in the sand by pushing an old credit card between the glass and the sand. The sand will look clean again.
Low nitrates and possibly stagnant water encourage it although it will form sheets in my skimmer box too. I just remove it best I can and have only resorted to drastic measures when it was a big problem.
You can cover the side of your substrate with electrical tape unless you really like looking at it. It's fairly easy and doesn't look too bad. My friend didn't even realize it was tape, she thought it was a feature of the tank.
If it is you can tell by the smell, it's very bad. I did the treatment with Myacin and you might loose some fish with any under lying illness, I lost 6 -8 Rummy's.
I had it bad but the treatment is listed in the Sticky at the top of the algae section, it takes 10 days but doesn't use too much Myacin and it works but I would wait and make sure that's your problem.
BTW it looks very thick and gelatinous but it can vary in color from blue, green, & red.