I've seen cases in the past when certain app icons appear incredibly small in the Dock, to the point that clicking on them doesn't matter. The icon on your screen is very similar to the Twitter icon, so if you are a Twitter user it might be.
Anyway, I would start with a reset of and restart of Dock. 1
To reset and restart the Dock, follow these steps:
- Go to Finder
- Press and hold the option key down
- Go now to Go menu
- Select the Library option to open the User's Library folder. You can release the option now.
- Go to the Settings folder (ie in the Library folder)
- Find and move com.apple.dock.plist file to trash
- Now open terminal (usually located in applications> utilities)
- Enter this command
killall -KILL Dock
- Press enter
Once you have done this, use your Mac long enough to know if this has solved the problem .