Yes, you are correct. The stage starts out dark and when it is lit up, we can see the pianist. A way of saying this is, "Spotlight on pianist." Or else you could say, "The stage lights up, and the pianist appears." Or even, "The pianist appears lit up on stage." Technically speaking a spotlight is a particular kind of light used in staging. However, if you write this in the script, the lighting technician and director can always choose to use a different light, they will however understand the direction.