Withe something growing

[Alcasrac]

Hello,
I found some withe stuff growing on the sump glass, when i turned off the return pump and turned on again so many this thing is came into the dt.
I have a week ago redsea algae management no3, po4-x.
I had some problem with the high nitrate level, now is going down.
Also i found thin “mucoid” stuff on the socks.
I think one of my fish is died, i didn’t see a week ago.
Please check the pics.

 

[Sean1986]

Looks like a type of sponge possibly

 

[Alcasrac]

After re-start the return pump:

So many small withe sponge peaces floing..
...that makes any problem? Over the filter socks, i have to clean every two days