Is warning someone in the mosque about their actions, which include sins and accusations of harassment, and not concealing their actions, considered a violation of the hadith: "Whoever does not conceal a Muslim, will be afflicted with what they have been afflicted with..."?

Whoever is not known for evil, and the default assumption is their innocence, should be shielded and advised, either as an obligation or a recommendation. It is not permissible to accuse them based solely on suspicion. However, if their transgression and evil are proven, there is no harm in warning against them to the extent that it repels their evil, such as permissible backbiting when necessary. Among the situations where it is permissible are: advising Muslims and warning against innovators and those who commit forbidden acts. For example, if you see someone associating with a transgressor or marrying one of their female relatives to them, you may mention what is wrong with that person as a form of advice, not harm. Therefore, mentioning a transgressor for what they are, in order to warn against them, is permissible and may even be obligatory. If what these individuals are accused of is proven and there is fear of corruption from them, there is no harm in mentioning it to the mosque administration. As for the mentioned hadith (definition): "Whoever does not shield a believer will be afflicted with what they were afflicted with," we do not know of it. And the hadith: "Whoever reproaches his brother for a sin will not die until he commits it," is not authentic.