Jesus is not saying here that he isn't "good". The point Jesus was making here was that God is good in the absolute and consummate sense. Jesus recognized God as the ultimate standard of what is good. (Ps. 25:8; Zech. 9:17) Though Jesus Christ had this quality of moral excellence, he would not accept “Good” as a title. That is why he said to the one who addressed him as “Good Teacher”: “Why do you call me good? Nobody is good, except one, God.” (Mark 10:17, 18) So Jesus was saying that no one is as good as God is, not even Jesus himself. God is good in a way that separates him from Jesus.
Mark 10:18 (and similarly Luke 18:19) are just two of hundreds of Scriptures where Jesus clearly referred to himself as a distinct and separate person from God. The Bible repeatedly shows that Jesus is not God but is exactly as the Bible said he was - God's Son. (John 1:18; John 10:36)
Mark 10:17, 18—Why did Jesus correct a certain man for calling Him “Good Teacher”? (w08 2/15 p. 28-p. 30 par. 9; Watchtower Online Library)
Why did Jesus correct the man who addressed him with the words “Good Teacher”? (cl chap. 27 pp. 270-279; Watchtower Online Library)
The Bible’s Viewpoint - Was Jesus Really the Son of God? (g 3/06 pp. 12-13; Watchtower Online Library)