Hi Hubert, et al.,
says that "U" is used to indicate a muted note. How to graphically notate a muted note may be different in various repertories/instruments (+ means mute in brass instruments, although it more means a stopped note with the hand in French horn music, which is the usual state for performance). In stringed instruments the graphical sign "+" means a left-handed pizz, and muted would be written out usually as "con sordino".
The circle following the note is not a harmonic, of course, but rather an instruction to unmute. (I see no marker on the above page for "unmute"). A general solution to unrepresented articulations is to use the !!!RDF**kern: system to assign a user signifier to mean something. A simple case could be:
!!!RDF**kern: O = unmuted
Where "O" means a generic ornament (borrowing for articulation, which is somewhat similar).