Transparency issue on printed document

Created a brochure with multiple masked photoshop images that are displayed on a grey background.

When printed (from Indesign or PDF) the transparent area of the images make the grey background darker, and so shows a square around the image.

Everything is CMYK, the mask is 100% black.

Is there a setting in Indesign that I'm missing that needs to change? How do I fix this 'ghosting'?