PSoC 5 can be a USB device connected to a host device such as a PC. It cannot be a host itself for other devices. So yes, in fact the only way a PSoC5 CAN be used is as a USB "Slave" device (I think the nomenclature for USB is Host/Device rather than Master/Slave).
So, if you had your PSoC 5 configured as a USB HID and connected it to your phone, provided your phone/app being used supports USB HID, it should work without issue, you would probably just need to have the phone in USB Host mode.
I imagine custom USB devices would probably be a whole other issue in terms of being supported by a presumably Android host. I suppose one could probably write software to support a custom device.