Can't see well enough using only the picture provided to be sure what they are.

Embo, the picture doesn't enlarge for you when you tap or click on it?

They sort of resemble peridioles from one of the bird's nest fungi.

OP, is there anything above this plant? Are these things adhered to the leaf surface?

Thank you both Embo and rhizo_1.

Yes these things adhere to the leaf surface, and if I try to remove them, it will come off but some white-color residual will remain on the surface (please see the picture) -- might these actually be bird's poop? I just notice that the big-root geranium behind the fothergilla also have the same thing on the surface of their leaves.

There is a large Norway maple tree maybe 10-15 yards from the fothergilla; the forthergilla is not covered by the shade of the maple tree but close to.

I think GG could be onto something. They're flattened disc shapes with dished centres. Just like peridioles. Maybe the fungi are in the mulch.

Thanks floral_uk -- indeed during spring and early summer there was a lot of mushrooms on the mulch near that spot, and one of the fothergilla was planted right above a dead tree's root..

rhizo here and not GG. ;-) OP, I doubt that you would recognize the cluster of bird's nest fungus, unless you did a bit of research. They don't look at all like "mushrooms". That you've noticed the disks on other plants emphasizes the peridiole theory.

I'm wondering if I see the little saucer shaped fungus in the surrounding mulch?

Sorry, Rhizo. my mistake.