Yesterday I noticed that my boy's poop was more liquid than usual. I thought he might just have been drinking a bit more than usual.
Today though he pooped on my leg and I noticed that the poop was a bit slimy?
In terms of behaviour, I know that cockatiels are good at hiding illness, but he is literally the portrait of health - very lively, attentive, FULL of energy, bright eyes, shiny feathers etc. He is 10 and shows no signs of aging whatsoever (lucky boy!)
Should I be worried and what could have caused the slimy poop?
Thank you!

Has he been eating anything unusual lately? If the poop continues to be abnormal it's best to call a vet. In the meantime, you could add a few drops of vinegar to his water. That can be helpful with minor problems.

Thank you for your reply.
He had some snow pea sprouts last week, which he seemed to really love, but in the last few days, no, he did not have anything new to eat.
He is still sleeping right now, but of course I will pay attention to his poop when he wakes up.
Is apple vinegar OK to add to his water? I have heard it is usually considered very healthy. I had it for quite some time, but I don't think it expires?

Any kind of vinegar will work. Apple cider vinegar is hyped as being the best, but it doesn't really make any difference since they all have the same amount of effective ingredient (5% acetic acid) and not much of anything else. As far as I know, vinegar doesn't go bad.

Thank you. I looked in the cage and I cannot see any strange poops today.
And as I said yesterday, he is very well and full of energy.
10 years old and still behaving like a boisterous youngster. He has actually never been sick in 10 years. He is a strong boy!
I think a drop of vinegar in his water is good for him anyway, so I will put a drop in his water.

The only thing is that he is still a plucker, and he has been for years. Just his personality. He is too lively and boisterous to live in a room I guess. I have given him all possible forms of entertainment, but he doesn't seem to be able to give up. Then again, his mother is still biting her nails as an adult, so I guess we are buddies.

Some birds just pluck, there isn't always a reason for it that we can see.

A small amount of vinegar in the water from time to time won't do any harm and might be beneficial. Doing it every single day is not such a good idea but I don't think you're planning to do that. Since he's given you reason to be a little bit concerned, it wouldn't do any harm to add a little vinegar for two or three days until you're absolutely sure that it has passed.

Maybe once a week?

He seems to be really well right now. He is dozing off on my knee, and the only reason why he can't really fall asleep is that he can't stop grinding his beak.

If you wanted to do it once a week on an ongoing basis it shouldn't cause any problems.