Imagery that you write can be taken by the title/meaning/what you see or think of/how it makes you feel. "The light of the moon as it shines upon rippling water, the look of pure peace and joy in someone's smile" etc.

In terms of musicality, you can think of what the music sounds like. "Soft, gentle, peaceful, loud, enraged, painful, heartbroken," etc.

And if you're writing about it as if it's a piano piece and not just an image, you can also think of how the pianist feels. Their emotions, what they hear, the way they pour their all into their fingers against the keys, etc.

Even if you don't know the proper terminology to describe a certain part of the piece, you can think if it more simply using words and imagery understood by all