The command line you are showing does not enable the grpc plugin. You need to specify an _out argument for the grpc plugin, which enables the plugin and specifies where it should output files. Since the plugin is likely not in your PATH, you also need to tell protoc how to find the plugin with --plugin.
So you need to add two arguments:
--plugin=protoc-gen-grpc-java=path/to/protoc-gen-grpc-java --grpc-java_out=path/to/output/dir
For more info, see the gRPC compiler documentation.