The photos show unremarkable white matter, and do not reveal any cystic lesion or abnormalities. In fact, I cannot confirm the tissue is septum pellucidum at all. Usually a cystic lesion at or near septum pellucidum is a colloid cyst lined by ciliated or mucinous (or both) columnar epithelia in a single layer. Neurosurgeons usually can take it out easily after fenestration and evacuation of colloid or mucinous content. Patients usually present with obstructive hydrocephalus. 

 Is it true cystic wall, or a mural nodule? I prefer to think of it as somehow degenerative tissue of septum pellusidum, not a tumor.