how to hide a cast while performing?

Dave Hill, the lead guitarist with Slade, broke his leg by falling off his platform shoes (really!) in the 1970s. He did a tour wearing a silver suit and wrapped the cast in aluminium foil!

Jimmy Page once broke his little finger and did a Led Zeppelin tour with his left fore-arm in plaster. He had his shirt sleeve closed over the cast with an elastic band and covered the bit of cast that stuck out with mascara.

Your question raises significant curiosities. Performing what? Cast on what?

If you're dancing the role of the Nutcracker and it's a full body cast, you might have more problems than if you're playing a Mendelssohn violin concerto and you're nursing a broken ankle.

Anyway, give consideration to costume adjustments, if it's a play or musical you're appearing in. For instance, a cloak or a robe could cover a lot of plaster. Also, changing the blocking could help. Spend more time behind this or that stage dressing.

And, if it's a stand up musical performance, just don't worry about it. Sit down, if necessary. Make sure the cast is washed clean (or painted presentably), and make the best of it.

(Maybe in this case it's wrong to say) BREAK A LEG!