Why does Nexus 5 HCE respond to FELICA select?
Besides HCE mode, an Android device typically also operates in NFC peer-to-peer mode. Passive peer-to-peer mode (ISO/IEC 18092) is based on the anti-collision and activation protocols from ISO/IEC 14443 (for 106 kbps) and from JIS X 6319-4 (for 212 kbps and 424 kbps). Hence, devices in peer-to-peer mode may be detected as JIS X 6319-4 cards as well as ISO/IEC 14443-3 cards by contactless smartcard readers.

How likely it is to (accidentally) detect an Android phone as "FeliCa" or to (properly) detect it as ISO/IEC 14443 depends on the implementation of the NFC polling/listening loop that the phone uses to cycle between the different active and passive NFC (and NFC-compatible) protocols.