Algae breakout from tank cleaning

[Nami]

I upgraded to my tank (50g)3-4 months back, transferred all my livestock and a portion of the rocks. Put in new sand as substrate.

In that 3 months, I didn't do any water change - only maintenance I do is cleaning skimmer, filter, dosing and wiping the glass. Overall, the tank was almost pristine with happy livestock.

Almost two weeks ago, I decided its time to do a small water change (10g) and vacuum the sustrate. The following week, brown algae started coming out of nowhere and grow on everything.

As of now, I'm still trying to manually remove as much as possible, my corals look ticked and not opening much.

What actually happened? Do I just keep removing as much as possible?

EDIT: I bought filtered seawater from LFS to do the water change. Even its not pristine, is it possible to cause such a algae breakout?

 

[Hitman]

I would say there had to have been a bloom or at least something in that water you got if that's the only thing that's changed. Did you test the water before adding it to your tank ?

 

[Mastiffsrule]

Sorry your having problems.
My first 2 guesses would be diatom from the new sand but it does not look like that. My second guess may be flow. New tank, maybe the flow needs to be dialed in more?

And I know this is standard question , where are you at on tank parameters?

Just a thought.