What client (curl, Ratel etc ?) are you using to connect to the Alpha Instance. And if it is a browser, then which one? Version?
My presumption is that the client does not trust the Dgraph Root CA (ca.crt) and hence this error.
(I see that you have added this to the KeyChain as Trusted Root, but some clients may not use the system Certificate Authorities).
Please also try the trouble-shooting steps here.
https://dgraph.io/docs/deploy/#troubleshooting-ratel-s-client-authentication