What is it called when a poetry stanza alternates between iambic tetrameter and triameter?

This is called 'Common Metre'if it is repeated once, that is to say if the lines are : 8,6,8,6.

The Lord's my Shepherd, I'll not want.
He makes me down to lie
In pastures green : he leadeth me
the quiet waters by.

[Psalm 23, the first stanza. The Scottish Psalter 1929.]

Common metre or common measure 1 —abbreviated as C. M. or CM—is a poetic metre consisting of four lines that alternate between iambic tetrameter (four metrical feet per line) and iambic trimeter (three metrical feet per line), with each foot consisting of an unstressed syllable followed by a stressed syllable. The metre is denoted by the syllable count of each line, i.e. 8.6.8.6, 86.86, or 86 86, depending on style, or by its shorthand abbreviation "CM".

Wikipedia - Common Metre

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Just out of interest, the other forms which are usually met with are short metre 6,6,8,6 :

To thee I lift my soul :
O Lord, I trust in thee:
My God, let me not be asham'd,
nor foes triumph o'er me.

[Psalm 25 first stanza, The Scottish Psalter 1929]

. . . . then double common metre (which simply doubles the lines to eight lines), then long metre 8,8,8,8 then double long metre (doubling to eight lines).

What @Nigel J said: common meter. I just wanted to add another example of common meter to try out something I just learned from @Andrew Leach's edit of @Nigel J's answer, how to single space:

"Because I Could Not Stop For Death"
by Emily Dickinson

Because I could not stop for Death –
He kindly stopped for me –
The Carriage held but just Ourselves –
And Immortality.

We slowly drove – He knew no haste
And I had put away
My labor and my leisure too,
For His Civility –

We passed the School, where Children strove
At Recess – in the Ring –
We passed the Fields of Gazing Grain –
We passed the Setting Sun –

Or rather – He passed Us –
The Dews drew quivering and Chill –
For only Gossamer, my Gown –
My Tippet – only Tulle –

We paused before a House that seemed
A Swelling of the Ground
The Roof was scarcely visible –
The Cornice – in the Ground –

Since then – 'tis Centuries – and yet
Feels shorter than the Day
I first surmised the Horses' Heads
Were toward Eternity –