http://www.cider.org.uk/part5.htm
"Mousiness is unmistakeable to those who have tasted it, although individual sensitivity varies widely from person to person. The flavour is best likened to that of a mouse-cage, although some people think it is closer to bread or freshly-baked biscuits. Strangely enough, the chemical compounds responsible for all these flavours are identical! This defect arises from a slow microbial action in storage, and although its chemistry is generally understood nobody knows why it happens in some ciders but not in others. Unfortunately there seems to be no reliable way of preventing its formation or of removing it once formed, although it is generally less common in ciders which have been sulphited, and access to oxygen appears to encourage it. "
If that doesn't sound like it, read though the rest of the page and see if anything does sound like it.