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

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).