They are harmless. There are capillary beds in the gill fimbriae. Sometimes, overt regeneration results in a thickened fimbriae loop being formed.
In some instances, blood pooling/congestion in a gill can also cause a distention to the vessel. There are no 'valves' in the gill vessels so its not the same as 'varicose veins' but the concept of increased pressure/congestion in a vessel causing distention is similar.