If you got it from a nursery, you'll want to wash it - the stems, branches, both sides of every leaf.
But as japanese people eat the leaves of these maples (Acer palmatum) in certain dishes, they are likely not toxic for chameleons.
I use canadian and japanese maple leaves in my roach gutload on occassion. Red maple (Acer rubrum) bark can have a fungus that isnt good.