Sorry for the typo, I meant "rope like"...sort of a stripe you can see on the two largest strings on the left.
Can you post a sample image? Would like to see it. It could be something with the lights, a reflection, flare, glare, shadow, but my guess is it is moire.
EDIT: now I suddenly see the sample image. Looks 100 % like moire to me. That happens when some small repeating grid-like pattern (string surface) is overlaid on another grid (sensor) and the grid spacings do not perfectly align. And in the real world they never do.
The reason is that the lens outresolves the sensor, and the sensor has no anti alias filter. You could deliberately misfocus your lens by a little bit, that would take care of it.
You could also try post processing software to first get rid of the color moire and after that use some light blurring to get rid of the luminance aliasing.
Also by shooting from a bit further away and subsequently cropping the image would very likely get rid of the moire.