Query on Photoshop

I'm sorry, this is not going to be helpful, but we usually think those type of artefacts are display issues caused by GPU driver problems, and not baked into the image.  i.e. you can save the image with its blue squares, open and view in another app, and the blue squares not there.

My first thought now, is that copy merge went wrong, and that if you turn off the copy merge layer and turn off each layer below it one at a time, the cause will reveal itself.  That does not have to be a raster layer, but if it was an Adjustment layer I would expect it to reflect the quares in its mask.

If none of that is what is going on here, then I am stumpped for now.  A full res screen shot of your entire workspace showing the layers panel might help.

Hey!  Are you in New Zealand?  I'm at Rapaura near Blenheim.

You may also post Help > System Info.

in the future, to find the best place to post your message, use the list here, https://community.adobe.com/

p.s. i don't think the adobe website, and forums in particular, are easy to navigate, so don't spend a lot of time searching that forum list. do your best and we'll move the post (like this one has already been moved) if it helps you get responses.

<"moved from using the community bugs">

Could you please post screenshots with the pertinent Panels (Toolbar, Layers, Options Bar, …) visible? 

Did you save the layered file?