The speaker looks back over a working life and finds little to be proud of. His knowledge and insight, once valuable, seem to be rusting, just like the older colleagues around him. He feels worn down and powerless: he can't change the way things are done, and the gap between what he wants and what is possible has drained his will to keep working. All he wants now is rest.
The title, "Leeches," only becomes clear near the end. The "hollow" and "hallowed pond" suggests an institution that was once respected but is now empty, and the speaker blames "self-indulgent elites and idiots" who keep sucking at it, generation after generation. His own ambitions and achievements, which should be a source of pride, only "ache the soul" and echo emptily.
The mood is weary, bitter and resigned. There is anger in the poem, but it has turned into exhaustion rather than protest.