On Illustrator CC 2017, after I turn on smart guides (CTRL+U) when I move the cursor over an anchor point it shows a strange small gray shape rather than the green text that was shown in CS6.
How can I fix it?
I can't really replicate the issue you seem to be having. Perhaps check the Anchor/Path Labels
option is checked in your preferences - see screenshot below.
In AI CC 2017, my default colour for the Smart Guide labels are magenta.
However the colour can be changed in Preferences > Smart Guides, to any colour you want.