Is that the back of the fabric?
I just went threw this with my machine, it was a flipping nightmare!!!! My machine would sew fine but when I went to FMQ it would give me this crap on the back!! I found that it was the machine hook timing. Had to take it to two different places to finally get that fixed. I had tried to use a needle that I didn't realize was too long for my machine and it got jammed and that's what threw off my hook timing.
It sewed fine for a long time and then it started to do it again!! But this time it was my tension take up spring had broken!! When your sewing along fine and then something suddenly happens to the sewing, it's not your fault, it's something mechanical.
Good luck and let us know!!