the hidden entity "hidden-electric-resistance" can take damage through tiberium growth. It's not supposed to be intractible, except by drawing a tiny amount of power.
it's type is 'electric energy interface'.

If that mod doesn't want them damaged, it should make them invincible.

We're not adding a hardcoded list of excluded entities because mod authors don't know how to make hidden entities work.

I was aiming more to adding a table that other mods can add their own entities that they want to have immunity to tiberium to.

We have one already.

Regardless, if they want their thing immune to damage, they should be making it so. It is as simple as setting the entity health to 0.

won't that display the health bar?

No more than the entity having health would.

The entity prototype needs to have 0 health, not the placed entity.