what is the correct grammar for the saying 'past/passed all that'?

We are past all that. The reason is that pass and past have different meanings.

You can pass a test. In the past you passed it.

Pats means something that has happened. Passed is part of the verb 'to pass' :-

I pass

You pass

he passes

she passes

we pass

they pass

Once you have done it you have 'passed'.

O have passed the last check point.

He passed the exam.

In your example you are using the word 'past'as something historical that you don't do any more, in other words you have progressed, so are 'past' all that.

The correct word is past. As in: past, present and future.

The other spelling 'passed' means overtook, went by. ie 'I passed some shops while driving my car'.

Edit - to further clarify, past refers to time and passed refers to space.

We are past all that.

As Bluebell suggested, in this sentence "past" is a preposition. Examples:

We are AT that point.

We are going TO that point.

We are PAST that point.

Passed is the past tense of pass. Example: The driver ahead of me signalled to pass him, so I passed him.

I'm pretty sure in the context you're saying it, the meaning is "past". When saying "passed" the meaning is more generalised whereas "past" is along the lines of time etc.

Hope I helped!

'past' sounds better but I can't explain the rule. Look at how the two words sound in the sentence

'we are past all that'

'we are passed all that'

Just to add my weight to Bluebell's: in this context, "past" is a preposition. It means "beyond" - "We have got beyond all that."