As on the title, is it possible for a moderator to modify or delete comments?
Yes. Moderators can modify and delete comments.
Comments can be deleted since comments are intended for improving a post, not for extended discussion. When the comments have been taken action (e.g. integrated to the post), then the comments are obsolete and ideally should be removed to clean up the thread and prevent confusion for future readers. Also, just like a post, when a comment is not aligned to the Code of Conduct, then big possibility it should be deleted. Flagged comments will be reviewed by moderators who can then delete them or leave them. (It's also possible for a comment to be automatically deleted if a certain keyword is triggered, or if it collects enough flags). See also: How does comment voting and flagging work?, When should moderators delete comments?
While moderators can modify comments, it is usually for an exceptional case, like salvaging a worthy-but-a-bit-rude comment, or fixing formatting when converting an answer to a comment. See also When should moderators edit comments?