Perhaps the easiest way to define the phrase is to dissect the phrase, define the words individually, and then place them back together within the context of the poem.
First, ethereal refers to something so perfect (delicate and light) that it seems too perfect to exist in the world.
A minstrel is a medieval singer who would recite poetry for entertainment.
Placed together, Wordsworth is detailing a man whose person or voice seems too perfect to exist in the world.
When placing the minstrel into the context of the poem, one can conclude that the subject of the poem, a skylark, is admired to a point above all others. Wordsworth exalts the bird in the poem. He does this by describing it as something not of this world, something whose vision is limitless, something which can withstand the flooding of the earth.
Always remember to take the title of any work into consideration when trying to interpret any part of a poem. The use of the phrase "ethereal minstrel" to describe the skylark places the bird on a pedestal showing Wordsworth's honor and mystification of it.