Do spells hit Hidden Tesla when they are underground?

No, hidden teslas will not be damaged while hidden. From the clash-royale wiki - "When it retreats underground when not attacking, the Tesla is unaffected by Spells, such as the Rocket and Lightning. It also cannot be targeted by cards that have a longer range than the Tesla, such as the X-Bow or the Princess, while hiding."

I read this question the other day and noticed this tip while playing Clash Royal so I took a screenshot.

As long as the tesla is hidden underground it will not take damage.