Helping Doctor Best

Mr. Frank, a 64-year old former postal worker was glad that "the worst was over". He was not very excited finding out that his mitral valve was leaking and that heart surgery was necessary. But it had to be done since he was already experiencing shortness of breath.
He felt very relieved now that surgery had been performed. His surgeon told him that everything went just fine and that he can consider himself the "proud owner" of a mechanical bileaflet valve. "Great" Mr. Frank thought to himself "what's there to be proud of? Well, at least I made it through surgery." Only 4 days later he was transferred back from the surgical ICU to the intermediate care unit.
Mr. Frank was now in his bed thinking of where he will spend his first vacation after successful surgery. Little did he know that his mood would suddenly change when Doctor Best, a cardiologist suddenly popped into the room dragging an echoscanner along.
"Mr. Frank, sorry to bother you but this is a routine echo scan. I want to see if everything is okay with your valve. I need to get some baseline measurements."
Doctor Best placed the transducer on his chest and suddenly his expression turned grim.
"Mr. Frank, there is something wrong, I am not quite sure what it is, but one question: do you have fever?"
"Well no....not that I know of. What's the problem?" asked Mr. Frank.
"You see - there is a mobile structure in your heart and I am not sure what it is."

Can you help Dr. Best?

Here are the images of Mr. Frank's heart: The mechanical mitral valve was functioning well, septal motion is abnormal and left ventricular function is reduced. But the most striking finding is a mobile structure attached to the subvalvular apparatus. But, what is the etiology of this mass?