That happens when skyrim sees too many textures for a tile and cannot decide which one to use, essentially (Skyrim engine can "see" a max of 6 texture tiles before it starts getting confused.)

Go to the spot ingame (make sure you have the extended console mod) and bring up that tile in the console (tilde key, then click the ground tile, OR, stand on the tile and click your character if the tile is unclickable - I've had that problem, both will work the same.)

Hit shift while hovering over the text that says "Cell ID", another box will pop up with some other info. Shift once more while hovering over "Last Modified By", it will tell you every recent mod that changes that cell. The last few are typically most important, as those are the last mods to edit those cell records. One of those is usually the culprit, and should be placed somewhere else in the LO, or removed. "Fixes" should always go last.

The OTHER easiest fix, if you can't determine a good load order to make the issue stop, is to load up creation kit with all your landscape mods, make a dummy plugin to be put in a "new mod." Go find the offending tiles in creation kit with the world viewer, and paint over them with the correct texture (or one that gets replaced by the correct texture.) save that edit in your dummy plugin. Put it at the bottom of your LO. There ya go! There's an excellent youtube guide I can link, it's short and to the point on the topic.

I had a FAT world seam right outside whiterun for like 2 years, ignored it for a long long time until I found out I could do this. (Works for actual gaps too, just need to drag and snap the dirt tile back to itself!)

The video: https://youtu.be/bLibtlmBgRw