The white ball icon is the default generic icon while the red cross was a screen shot preview that was automatically generated and is still being referenced even though you have deleted the actual mesh it was created from. This is harmless and unless it occurs frequently enough to be able to reproduce the issue, it will not be addressed.
If the red cross icon is confusing or causing work flow issues you can simply delete it. If it contains nodes you want to keep, you can copy and paste those nodes into a new, “fresh” blueprint and then delete the blueprint that is still using the cross for a preview.
One final thought, you may want to add a new mesh to the blueprint and hit save. This should result a new preview being generated from the new mesh.
Please try one of these resolutions and let me know if this continues to be a problem for you.