NoIdeaWhatThatIs I don't understand your problem.
When you use the "Quote" button provided by the Mentions extension, it will inject the quoted text formatted as a Markdown mention, with >
in front. I don't think this can be changed. If you have disabled the Markdown extension it will still insert the Markdown syntax, but Flarum will not render it as a quote block, so you'll see the ">" verbatim.
Please include your php flarum info
output and if possible, a screenshot of your problem.