From what you state, either the current or prior owner should pay. If the pipe solely services the single unit, I fail to see how that becomes an Association problem. Generally the inside of the Unit is the owners responsibility, and the exterior services are the Association liability. The CCRs generally lay it out pretty clearly; read them carefully and your answer may become evident.
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The facts as I understand them are (1) the pipes service the ground floor and the 2nd floor unit of this complex and (2) it is the ground floor unit that has suffered mold as a result of the portion of plumbing which runs through the ground floor unit to the 2nd floor unit.
Typically, but not always, the infrastructure (plumbing; electrical; gas, etc.) located inside the walls is the responsibility of the HOA. Since the HOA repaired the visible plumbing leak, then I assume the HOA has acknowledged some responsibility for the infrastructure located inside the walls.
The allocation of responsibility for the wall interior infrastructure should be, but is not always, set forth in the Declaration of Conditions, Covenants & Restrictions ("CC&Rs"). If the responsibility is allocated in the CC&Rs, then that allocation of responsibility controls. Other documents to review (1) Condominium Plan; (2) Rules & Regulations; (3) Maintenance Agreements: (4) or similar documents.
If the costs of the damage is less than $10,000.00 and the HOA refuses to pay for the damage (after you have reviewed the documents listed above and have determined HOA responsibility), then an alternative relief to obtain reimbursement may be small claims court.
Unfortunately, without reviewing the relevant documents, this response can not be a simple yes or no answer.
This response is not legal advice and is based solely on the limited information provided. You should contact an attorney or the Bar Association to get additional information and representation. This response does not create an attorney-client relationship.