Update (9 Oct 2015): It has been confirmed it was the filming for their upcoming Japanese MV at a handshake fanmeet.
There were sightings of Crayon Pop in the midst of filming an MV in Songdo, Incheon. They wore striped shirts with pinafores along with caps in their individual colors. This filming is the second day of their private schedule. It is still unknown what MV this is specifically for.
Although there is a Japanese schedule at the end of the month (for which they are likely to do the Japanese MV and photoshoots), it cannot be ruled out that this filming is for the upcoming Japanese single album in mid-November.
Source: Instagram 1, Instagram 2, Instagram 3, Instagram 4, Instagram 5, Instagram 6