I’ve searched the forum but don’t see this issue reported.
When I open a file in writer that has been created in MS word I have the occasional spaces that are grey - see image here:
The only way to get rid of them is to delete and retype a space, which is not practical for a large file. Find / replace doesn’t work for this.
The grey is not a highlight / fill colour - I’ve tested this by highlighting it and picking no fill, and it doesn’t change anything.
It’s a non-breaking space, intended to keep, in this case, the date on one line. You can turn off the gray shading by toggling View - Field Shadings (Ctrl+F8). It won’t show when you print the file. You can enter such spaces with Ctrl+Shift+spacebar.
Also see this one: I have gray marks between the letters - how get rid of?
This answer is the good one, unlike the ones under the linked question that say to replace non-breaking spaces with simple spaces. That is not a good solution, and not a solution at all, as non-breaking spaces can appear again if the case in newly edited documents, as this is not just related to MSWord, but can come from LO too, although not in English. For example if you write in French « mon frère » those extra spaces that do not exist in English will automatically appear.
This is a great answer - since it also explains how to insert nbsp and hence how one could search-replace.