Hi @xiaohua_yang and @ludwi Welcome to the Forum!
As it currently stands, it is not possible to hide completed tasks (or customize its font) in the Calendar view. This is a popular request in the Forum: Calendar - HIDE completed TASKS . I would recommend you to add your vote to support this request.
In the meantime, a workaround could be to run an Advanced Search to see only incomplete tasks of a particular project. You can then save that search as a report and use the Calendar of the report. Please see the screenshot attached.
I hope this helps! Please let me know if you have any follow-up question! Have a great week!