30.0 mean "variable".
If you click on the "Ptm" column header, you get a pop-up window including "Note (2) : codes used for the magnitudes:"; below that is a list of codes, including "30.0 = variable (var. in published catalog)"
FK5 is based on optical observations. ICRS is based on radio data. Radio observations (ironically) are much higher accuracy than optical through the application of long baseline interferometry.
Other than extremely accurate astrometry (fractions of an arcsec), they are essentially identical.