greenscreen keying technique

you may be seeing keylight’s despill efforts, where it tries to remove any green screen color that spills on the subject from the screen. this often looks a bit like noise that gets added to the subject.

to double check that that is the issue, toggle the ‘view’ from final result to intermediate and see if the problem persists (at the possible expense of green spill on the subject).
if you still want to suppress spill but try and lessen then noise that the spill suppression is causing, often setting the replacement method (in screen matte settings) from soft color to hard color can help.
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it essentially uses 2 instances of keylight, the first to create a core matte (it calls inside mask), the second to produce the key.

another good technique is aharon rabinowitz’s ‘super-tight junk mattes’ technique that you can find in the ae tutorials here at the cow.
and you can use both techniques on the same key to really define the area that will be affected by keylight.
for more complex spill suppression situations, andrew devis also has a tutorial here.
you can also find several techniques and tutorials here (and on the web) for creating light wrap when you are compositing your keyed elements into a shot/scene, which will usually really help to make it look natural.