If fine indoors , but curled damaged leaves when out on the balcony it is almost certainly caused by thrips sucking out the juices from the leaves. Thrips damage normally is on new young growth, and not so much of firm mature leaves. You can kill the thrips by a horticultural oil spray, or ever a soapy water spray. Spray both to upper and lower portion of the leaf. To avoid the damage, you should spray all new growth every 3 or 4 days until he leaves firm up.