To reproduce: Add a Drums contraption. Add any contraption that has combo-boxes in it (for example Bassline). Place the Bassline over the Drums, so that one of the combo-boxes is positioned over the Drums' pattern editor. Press play. Move mouse cursor over the combo-box and wait. When the vertical line that runs through the Drums moves underneath the bassline combo-box while the cursor is positioned over it, it leaves weird green dots visible around the combo-box :)
Happens on a Lenovo notebook with Sandy Bridge intergrated graphics (HD3000), Windows 7.