We use the word "seeing" when it's a gerund or verbal noun:
Seeing the mistake, she corrected it immediately.
I remember seeing her.
Fancy seeing you here.
Seeing is believing.
when you see something happening over and over:
I think that explains why recently I've been seeing a chicken in our yard.
I've been seeing things that aren't there.
or when it means something other than perceiving, like meeting or dating:
Are you seeing someone?
Jim started seeing Kate last month.
I'm seeing the same therapist for many months now.
But if you use it with the meaning of perceiving with your eyes and use it in the continuous tenses, it's usually considered wrong:
I'm seeing a plane in the sky!
Instead we use:
I see a plane in the sky.
I can see a plane in the sky.
There's one exception I can think of. If we modify my sixth example sentence, we get:
I'm seeing things that aren't there.
But even in this case one could argue that using present simple would be better.