This article is not clear. I'm not sure if this issue is directly related to the planned waste water release since that hasn't happened yet. Here's a few scenarios off the top of my head:

1. Ground & rainwater continue to seep into the power plant and become contaminated. Maybe some of this water is not collected and instead flows out into the breakwater, continuously carrying additional contamination with it. In this case the release or non-release of the separately stored & treated water is not related to this issue.

2. The stored water is leaking out and contaminating the breakwater area. In this case the contamination could indicate that releasing this water is a bad idea because it will release more Cs contamination. Or maybe not, because I don't think the currently stored water has been treated yet for release. The treatment plans for the stored water includes filtering out Cs.

3. The contamination in the breakwater is leftover from the initial disaster a decade ago and not new. Cs-137 has a half life of ~30 years, so most of it would still be around and it's known that a lot of this stuff got into the seabed sediment in the area. In this case it again would not be an indication about anything related to the planned water release.

4. Some other scenario.

Unfortunately there's just not enough information in this article to say for certain what the origin of the Cs contamination is and what consequences that holds.