Four short lines, and the whole poem is a single picture: a fountain, described as if it were a living thing. The water becomes an "aqueous angel" with a "silver song," so the sound of splashing turns into something close to music, even a kind of heavenly singing. The word "aqueous" is spelled differently in the text supplied ("acqeous"), which looks like a typo, but the meaning is clear.
The most curious word is "pouting." A fountain's spout pushes water out like lips pursed, and the poem plays on that idea, giving the fountain a slightly cheeky, human expression. "For always" suggests it never stops, and "imperturbable" adds that nothing disturbs it.
The speaker doesn't state a feeling outright, but the tone feels admiring and quietly amused. The mood is calm, bright, and a little playful, like pausing beside running water and noticing how steady and beautiful it is.