It can be hard to tell from a photo, but I'm pretty sure that's not diatoms. I'm less sure whether it's dinoflagellates or not. But it could be. Mine always seemed to want to grow toward the surface like it wants to float (due to gas trapped in it). And if I stirred up the tank it would break up and make a huge mess.
I'd drain the sump, toss the water, wash the sump out with something that kills, like Hydrogen peroxide, vinegar or dilute muriatic acid (at 10:1 it's cheaper than vinegar and can be made stronger if you need it). Rinse it good, put it back in the stand, fill it with new saltwater and 1ml of hydrogen peroxide for every 10g of water in the sump and let it sit for 30 minutes. Then fire up the pump and let it go.
Dinos can be really tough to get rid out of the system. I had to do this kind of treatment 3 times in 4 or 5 weeks. It's now been 6 months with no issues. BTW, 1ml of hydrogen peroxide per 10g in the sump will help kill algae in the sump, but when flushed into the rest of the tank it will barely have any effect... maybe you'll only have to clean the tank glass half as often!
Good luck.
