Currently I'm on the lookout for public displays of sentiment towards the current political class here in Hong Kong. CY Leung, Hong Kong's current Chief Executive is the target of the graffiti, here referenced as '689'.
The number is used in reference to the number of votes that elected him into his office. The number is used to ridicule the lack of representation of the will of the Hong Kong people.
For reference - Guardian - 'Why are the Hong Kong protesters rallying around the number 689?', BBC - 'CY Leung: The troubles of Hong Kong's unloved leader' and Wall Street Journal Blog - In 'Hong Kong, a Democracy March with a Sense of Humor'.