It could be evidence of serious leakage, or evidence of a bad prep job, or both.
Last spring before I bought, I went through the house right at the end of a hard 3-day rain. The basement was dry, os I felt safe. Its been fine so far.
The block walls were never painted and one corner had some white powdery mildew near the floor. I used clorox and a brush to thorughly clean and kill the mildew. Hopefully, my prep work was good enough. The point is, if they painted right over the mildew, then its going to eat right through laytex paint feeding on the paint and look bad in a month or two.
You must take the evidence you see seriously and evaluate its impact. Leaky basements can be repaired. There may not be much or any current leakage associated with small amount of mildew. Ask the seller and try to get honest feedback from him. Find out when he/she painted it, how they painted it, etc. Try to look at the house right after a hard rain, however you may miss out.
If the house in on the east coast, and you don't see leaking after the recent storm, I would say it would be safe.