TSA is a requirement for all non-us citizens. There is a difference between citizen and resident/green card holder. A green card holder still needs TSA clearance. A US citizen still needs to provide proof of citizenship, by using a birth certificate, passport etc for more info on that read 49 CFR part 1552
Now my understanding is that TSA is only required for initial training on
ASEL/S, AMEL/S and Instrument. So my conclusion is that you would not need one for ATP as you would already have your instrument.
BR
Johnny