Is this the look of your PDF when viewing in the proofing app?
Whats happening is a Moiré pattern is appearing due to not being in the correct colour space. If you gently pinch to zoom in/out on the affected page you will see the pattern changing.
From past experience with these (They are quite rarely seen) we've found the most effective solution is using a combination of Adobe Acrobat Pro and Photoshop.
This will allow you to remedy the problem, without compromising any loss of quality for the text or images, however if this method was employed when producing PDF's in their production phase they would not require this fix.
1. Using Adobe Acrobat Pro, extract each offending page separately using the Extract tool. Save each page individually as a PDF.
2. Once you have all your pages to work with, select the Edit Object tool available in Acrobat Pro and left click on the layer that is appearing as the Moire pattern. (The Edit Object tool is located Print Production, but this will not show by default. There's a little drop down symbol in the right hand corner just a little bit above Content Editing, seen in the screenshot to the left below.
3. EITHER right click on this offending layer and click Edit Object to send it to Photoshop (Photoshop may give an error message regarding incorrect colour space, so proceed using the other method) OR save this page again as another PDF. We will end up with 2 identical PDFs.
4. Presuming you now have 2 copies saved of your PDF, press the delete key whilst the offending object is highlighted (or right click > delete). This will delete the offending object from this copy of the PDF. Save this file as this will ultimately become the fixed version.
5. On the other PDF, use the Edit Object tool to delete all the layers BUT the offending layer from Step 4. Save this PDF with a filename like "background_pagenumber"
6. Fire up Photoshop and open up the background PDF, click ok to any dialogues that appear.
7. Without editing this file at all, immediately go to File > Save As > Photoshop PDF. Click ok to any dialogues that appear. You are now left with a PDF of the background layer.
8. Open the PDF with just the text/images and no background layer that you've just saved from Photoshop. Select the Edit Object tool and first left click on the background image to highlight it then right click > Copy.
9. Go to the PDF with just the text on and right click > Paste.
10. The background image will now be on the "top" layer so to speak. Right click on it and click Arrange > Send to back.
11. This page will now display correctly and you can use this one to replace the old one in the master PDF.