I think folk are scratching their heads as to why there is bulky ugly (grey plastic) fitting protruding into the room...?
What finish are you trying to achieve? Something flush? Something to match existing? I appear to be looking at floor tiles (?), tile backer (?) and floor boards, but I can't see the edge of the tiles (?) only the edge of the tile backer and floor board below (?).
As far as I can tell it's a soil pipe coming up from a wooden floor behind a toilet. You can't just pour filler in as the void under the wooden floor is open.
Have you got any tile offcuts? If not maybe you can get a single tile as a sample that is close enough. Then some careful cutting with an angle grinder should get you a filler piece. It'll bridge the hole ok as you can't walk on it. Make a template with cardboard to get the fit. Go a little undersized and you can fill the rest with silicone.
Of just a bit of wood painted black. Easily cut with a jigsaw.
A piece of ply, in two parts, with the two halves centred on that pipe, so it can be slotted in. Paint the ply matt black, glue into place with your sealant.