\$\mu\$ Symbol on a Relay Module's Nameplate
It appears to indicate “micro disconnection” with a physical contact gap that is not of a certain minimum size. From this Shrack/Tyco document:


  Micro disconnection
  Adequate contact separation in at least one contact so as to provide functional security.
  Note: there is a requirement for the dielectric strength of the contact gap, but no dimensional requirements.


It may be described in IEC/EN 61058-1, but I do not have a copy to confirm.