Quick summary on the Hong Kong situation: A couple of months ago, an extradition bill (ELAB) was proposed by Carrie Lam, the HK chief executive. It would allow criminals to undergo extradition between Taiwan, HK and Mainland China. This caused worry by HK citizens that China would use this bill to take basically anyone they don't like or who criticizes the party/government back to China on trial.
Since then, there have been massive protests in HK against the bill. The bill has been put on 'indefinite hold', but has not been withdrawn and the government is ignoring the protesters requests. The protests have turned violent and people are saying that the police are using inappropriate amounts of violence on the protesters. China is condemning the protests and there are videos/pictures of Chinese Armed Police masses and tanks queueing up in Shenzen, which is just across the border with HK. (If you want to read a bit more about it, here's the full twitter thread I used as a source. Also a video of recent protests at the airport.)
Since then, Chinese K-Pop idols (along with most Chinese celebrities) have been voicing their opposition to the Hong Kong Protests on weibo by joining 'The Five-starred Red Flag has 1.4 billion guardians'-hashtag campaign. (If this sounds familiar, last year there was a similar campaign -> Omona post.)
Guanlin, Cheng Xiao, Lay, Victoria, Kyulkyung, Jackson, Jun, The8, Lucas, Yanan and Yuqi all retweeted the campaign on weibo as support for 'One China'. (Lay also posted it on IG and Twitter).
There is also another post (that not all of the idols above retweeted) with "What a shame for Hong Kong" that says “I support the HK police. You can beat me up right now.” (This was said by a China mainland tourist/journalist at the airport, who got beaten up by the crowd after he said that). The hashtag says 'I too support Hong Kong police'.
Edit: Clarification from /u/Sweet-Lullaby @ r/kpop:
There are two posts that Chinese celebrities posted. The first one was about being protectors of the Chinese flag but I can’t find the translation. Every Chinese celebrity working on the mainland has posted that post including rookie idols like G-IDLE’s Yuqi and NCT’s Lucas, Pentagon’s Yanan and as well Jackson.
The new issue is the second post that Chinese celebrities are now posting which specifically states they support the Hong Kong police.
The Chinese Kpop idols who posted the Hong Kong police support post so far:
*Lay ( he went and posted on twitter and IG and not just Weibo)
*Cheng Xiao ( WJSN)
*Jiaqiong ( Kyulkyung)
*The8 and Jun ( Seventeen)
*Lai Guanlin ( ex-W1)
Some Chinese celebrities have managed to remain silent and not post either post on social media and so far managed to fly under the radar.
[trending] #ZhangYixing’s studio announces that the Hong Kong stop of his concert on Aug 17 is cancelled due to safety issues,in light of current situation. They’ve tried increasing security & other measures but are unable to ensure safety of concert goers, hence the cancellation pic.twitter.com/Ds3mlJ4L0w— c-drama tweets (@dramapotatoe) August 12, 2019
[trending] #ZhangYixing’s studio announces that the Hong Kong stop of his concert on Aug 17 is cancelled due to safety issues,in light of current situation. They’ve tried increasing security & other measures but are unable to ensure safety of concert goers, hence the cancellation
Use the #jype_cancelGOT7hkconcerts and tag the GOT7 and jypnation account to cancel the concert or postpone it, for the security of GOT7 and drown.
In weibo they have revealed the arrival time of got7, and they have threatened to burn the flag of China in front of Jackson or give him a flag with blood, also the concert is near the airport where at this time the protesters have 6 days that have it taken
source: @danveralkyrie, naver, netizenbuzz, dramapotatoe, blinksarmysvips, hk01, Sweet-Lullaby@r/kpop
Edited in a clarification about how there were two separate posts that got retweeted and what they mean.