That is a Unicode replacement character. If this character is appearing in your table then it might be that you are issuing queries using the wrong character set. You should check the column character set, and you should also check the character set(s) of the connection(s) you use to issue queries. If there is a difference in connection character set between connections used to read and record data, or if there is a difference in expected character set between applications/scripts used to access the data, that would explain the presence of these characters.

If you just want to replace it with a space: