Strange white stuff on my rocks and sand bed

[DanBeck]

Hello guys, I hope you are all well.
I’m very new to salt water tanks. I started mine about 3 weeks ago.
I have this white stuff growing on my rocks and sand bed.
I thought it might’ve been a sponge but it grows incredibly quickly and sticks my gracilaria algae to the rocks.
I also saw my one damsel picking on it which made it come off in a “puff” or like a powder

 

[Timfish]

I see that when excess food starts to decompose or on new maricultured live rock in quarintine when something has died on it. Siphon it off and if you're feeding watch how much your feeding to see if it's too much for your tank inhabitants to deal with. What fish, corals and other invertabrates do you have in your system?

 

[DanBeck]

Thank you for the info. I’m definitely feeding too much. I’ll scrape it off and see how it goes. Should I treat it with vinegar at all?
I have 2 ocellaris clown fish, 2 azure damsels, a hermit crab, a few small crabs, fan worms, clams and some algae.
The four fish are still quite small

 

[Timfish]

I wouldn't scrape it off. THat whaite stuff is a pretty thick bacterial mat and it's reducing the oxygen levels as well as producing a lot of hydrogen sulfide that can be easily released into the water. It's much better to gently siphon it out along with any residual food. This technique with steel straws might help: