If a helmet doesn't have durability I.E. Advanced rocketry's space helmet, a brain slime can destroy the helmet.

I guess a good solution to fix the Brain Slimes destroying unbreakable helmets would be to allow the brain slime to do damage to the player when it detects an unbreakable "helmet" currently being worn.

Yeh I thought I had a check for this. Apparently not. Thanks for the report

Should be fixed next update