Will the impurity that has stained the elbow—semen mixed with pre-seminal fluid—become pure if water reaches it during ghusl, but it is not rubbed?

Washing away impurities is sufficient when one's dominant impression is that they have been removed. Certainty is not a condition, because requiring certainty in this context would be an undue hardship that is negated by the Sharia. So, if one's dominant impression is that the impurity has been removed by water, then this is sufficient, and rubbing it is not necessary. However, if one's dominant impression is that some of it remains, then it must be removed.