This looks like a transparency flattening artifact called stitching. Where are you seeing it? Looks like in a PDF. If it is stitching, you can eliminate it byy exporting to Acobat 5 or higher compatibility and keeping the transparency live.
Hi there Peter.
The stitching is present both in PDF and in InDesign UI (snapshot is from InDesign).
The stitching appears when I've added one of the effects displayed in screenshot (drop shadow/inner glow/outer glow).
The stitching is most prominent having transparency effect set to Darken but also somewhat present in blending mode Normal.
Exporting to Acrobat 5 or higher does unfortunately not solve the issue...
If you turn off Anti-aliasing in the InDesign Prefs (Display Performance > Enable Anti-aliasing), does the stitching go away? If so, then you have nothing to worry about.
Similarly, you could turn off the "Smoothing" options in Acrobat prefs and check the file.
Can you post a sample file someplace?