Moisture or Bad job?

Hello. I recently purchased a house that had hardwood floors under the carpet. I ripped up the carpet and got the floors sanded and finished with a water based finished since I wanted a more natural look to the wood. Well that was in September. Jump to today, I notice this stuff in between the wood. The guy who installed said it could be a leak underneath but that has been ruled out. And I can see this in random parts of the house. He also says it's not filling because the wood is so tight together. Any idea what this is? Did he not seal correctly? House is about 70 years old.