Shadelight

Oh, I like that. Gives a feeling of 'they were already here' with a dash of 'your awareness hadn't picked up on them immediately - how strange...' However the taking-some-time-to-focus thing should be dependent on how close these rust monsters are to you, for balance reasons - nobody wants to fight a barely-discernable blob while it's charging at you at full speed. Monsters within, say, 5 blocks of your location should take less than a second to fully lose their blur. Monsters that are farther than that, though, can take longer, as it adds to the atmosphere.

But why not at least add a minor blur effect to when they spawn in? Or something similar? It would still be eldritch in vibes, but not jarring in a way that makes one think the game is still unfinished.

Seconded. Them just popping in feels... unfinished.

Still a thing. Kinda disappointing, since I'm using gates as openable windows.