If the OP wish's to escalate this to civil court remedy.

Old homes have their issues but everything can be remedied.

I was sick of paying for bed bug remediation.....ergh..I still sell to clients there. 

Make sure you take the old window out and remediate any mold problems before doing anything.

If so, what is the remedy?

At the same time, have you and the neighbors talk to the town and hear your legal remedies, the town will be on your side, especially if you have written proof that it's decreasing property values, they'll probably be on his tail just like you are and can help you enforce in less than months probably. 

I am sure it's also upstairs.... how easy is it to remedy this?

Also, it goes without saying that you should consult an attorney; Okay briefly reading over your lease agreement, it's a good agreement but it is very loosely written and does not provide for many remedies prior to eviction so... that is something you should look to improve on moving forward, there should be an escalation of force in as $XX fee until rectafied and THEN otherwise eviction, also you should number your lease clauses.Regarding the current situation first and formost, I understand you live in the property as well but you should certainly take on a 'HANDS OFF' approach in regards to internal disputes amongst Tenants; you are after all a landlord, not the ethics police nor the real police.... if other Tenants have issues such as those they should inform you and then they should contact the police... if it's not police worthy than it is more than likely just internal bickering. 

As remedial legislation, the terms of the Act are strictly enforced." 6404 PARK AVE., LLC v.

Professional remediation is best for larger or stubborn mold issues.