TAC has said that this is a bug in version 6.7 and an update is required.
This behavior shown is a cosmetic error and has no real impact on your devices. It happens because of depricated features on version 6.7
This is described on the bug: https://bst.cloudapps.cisco.com/bugsearch/bug/CSCvy24498
The documentation shows that full support for the health module returns on version 7.0.0. (Requiring both FMC and managed firepower devices on that version)
For more information:
FMC version 6.6 release notes: https://www.cisco.com/c/en/us/td/docs/security/firepower/660/relnotes/firepower-release-notes-660/features.html#id_122921