That is called "Eyelashes" and it is typical when your speed of stitching is not in synch with the movement of your hands. This is a very common problem with FMQ. There are tons of youtube videos as well as just articles about how to correct this.
I believe you are moving the fabric too fast for the machine speed. So either slow your hands down or speed up the machine. It takes a bit to get into the 'groove' of FMQ, where speed of stitch and speed of hands are aligned.
not to worry, you will get it. But feel free to google "Eyelashing with Free Motion Quilting" , you should get alot of visual and video results to help.