Advice needed for newbie

[Spezdoz]

Hi all,

I am fairly new to the whole aquarium set up but have been reading lots and watching lots of YouTube tutorials.
We recently moved our fish to a larger 2nd hand tank. Cleaned prior and such and have recently noticed this whitish fluff at the bottom on the tank by one of the ornaments and plants.
I'm not sure what it is and I do not want my fish to get ill. Water levels seem fine and amonia levels are fine (at 0).
Is it as simple as water changes at 30% along with sand clean or is or something more serious?

Any advise is appreciated 😊

[ADsnail]

Hard to see what's going on there with the fluff. Have you reached in to touch it, pulled a bit out to rub between your fingers to determine its composition? Who lives in the tank, what do you feed them? It could be leftover food that is molding, i.e. overfeeding remnants rotting. My experience has been pelleted food in particular does this. Could be other things, but I would start with removing some and investigating close-up and physical.