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.
