Does human!Mani still maintain and childlike belief in magic? That is, would he even be able to see non-human versions of his loved ones, or would he pass right through them as most adults simply pass through the Guardians and other such magical creatures?
He's a man of science. Mani realizes that what he sees can't honestly be all there is to the universe. So in his own roundabout way he does believe in other types of being. So seeing those who are alien life forms would be easy enough for him [Aster, Sandy, and to a degree, Pitch]
For others, it would be very difficult for him to see. He'd probably see them well enough with his eyes, but his brain would ignore them in the same way one ignores passing scenery while driving. Sure you see it, but it's not important that you notice it.