The phrase word of mouse is a new version of the older term, word of mouth, that applies to the digital era - word of mouse is people passing along messages about a company or product (or anything else) through digital forums.
Word of mouse happens in many ways. People can send messages through email, but they can also send messages through social media platforms such as Facebook, Twitter or others. Word of mouse can also happen around review sites or in the comment fields of articles. In general, companies cultivate a certain reputation online to promote good "word of mouse." It is all part of the expansion and enhancement of digital communities and forums in the twenty-first century.