Algae

[Dboyd6]

What is this yellow stuff? Is it algae or what it is on my coral and in my sand

 

[MSB123]

Its because your tank is still cycling. It is probably diatoms, I would pull your fish/corals out and let the tank cycle for a few more months, it generally goes away on its own when there are no extra nutrients (fish poop, anenome poop)