Emacs, How can I display only current task and hide others in org-mode?

You also have org-tree-to-indirect-buffer, or C-c C-x b, which will create a new buffer with only this subtree, and reflect any change in one buffer into the other.

Use narrow-to-defun or org-narrow-to-subtree command with point in task. (widen to move back to whole buffer content).

Default shortcuts:

C-x n s      # org-narrow-to-subtree (bound in org-mode)
C-x n d      # narrow-to-defun
C-x n w      # widen

Manual