It is common in cattle and buffaloes to see vesicles containing clear secretion on the serosa of the uterus. But your case seemed as old connective tissue due to local inflammation or irritation. Histopathology could confirm this suggestion.
I also agree with previous comments about need of histopathology. I believe vesicleson the serosa of the uterus could contain white condensed secretion, fibrine like, or fibrotic tissue.