Normally these slices appear when a transparent image which has been flattened is used in InDesign. The background comes through when vector antialiasing in Acrobat is turned on. These lines are normally white, in some cases, when something is behind or with a colored frame they can have another color.
It will happen when you place any form of flattened image in InDesign as there are PDF/X1-a, PDF/X-3, EPS, PDF without transparency as any PDF created with the distiller and more so.
Those lines are normally invisible in print, but when it comes to export to JPG or PNG they will become visible if you export to a different color space than the flattened image was before, e.g. image is CMYK in ECI v2 300 and you export an EPUB which will use sRGB (1966), or from one RGB, let's say ECI RGB to sRGB. (ECI is the European Color Initiative, close to a standard http://www.eci.org)
Could it be that you used an image with flattened transparency?
Use RGB images with live transparency.