Stand abilities are still getting deteriorated. How does a cut make them stand users? That alien was also scratched by that arrow, why didn't he get a stand?
@SSS26032002 Well they say the arrows has a virus on it that makes you gain a Stand when you successfully survive a cut. So since it's a virus or bacteria (or something similar) doing that, it may be that the alien was too different of a being for the virus to successfully work on it.