That's just the anti-halation dye on the back of the film dissolving, as it's designed to do.
It will discolor the first liquid used, whether it be pre-soak or developer.
This is absolutely normal. It happens with all films (except x-ray film), and is of no consequence whatsoever.
It's chemically inert and will not affect any photographic chemistry. The color will vary with the particular film in use, but it's irrelevant.
- Leigh
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