Is the Hadith, "Whoever harms a wali of Mine, I shall declare war on him," a Hadith Qudsi? What is the meaning of "wali" in it? And does its implication apply to someone who backbites or slanders a person whose outward appearance is one of piety and devoutness?

The Hadith referred to is a Hadith Qudsi, which is authentic and narrated by Al-Bukhari. Its wording is: "Allah said: 'Whoever shows enmity to a friend (wali) of Mine, I shall declare war upon him. My servant does not draw near to Me with anything more beloved to Me than what I have enjoined upon him. My servant continues to draw near to Me with supererogatory acts until I love him. When I love him, I am his hearing with which he hears, his sight with which he sees, his hand with which he strikes, and his foot with which he walks. If he asks Me, I shall surely give to him, and if he seeks refuge in Me, I shall surely grant him refuge. I do not hesitate about anything I am going to do as much as My hesitation about taking the soul of My believing servant, for he dislikes death, and I dislike causing him pain.'" A friend (wali) of Allah is someone who is knowledgeable of Allah, assiduous in His obedience, and sincere to Him in worship. Thus, every pious believer is among the friends of Allah. The meaning of "I shall declare war upon him" is: I shall inform him of it. So, whoever harms a friend of Allah, whether through backbiting or other means, has thereby made lawful Allah's war against him.