If I pass during some nocturnal blackness, mothy and warm,
It starts again with the hypothetical "if" – now the speaker is imagining what might happen if he should "pass," or die, on a warm summer night.
"Mothy" seems like a weird word choice – it reminds us of summer nights when moths are flitting around outside (or inside, if you don't have screens up), but it also sounds kind of like "moth-eaten".
When the hedgehog travels furtively over the lawn, One may say, "He strove that such innocent creatures should come to no harm, But he could do little for them; and now he is gone."
On the warm summer night described above, the speaker imagines a "hedgehog" sneaking ("furtively") across the grass.
Why is the hedgehog being "furtive"? Well, it's a wild animal, and it's on a human's "lawn," so it's probably in danger of being trapped or shot at or shooed away.
The next line suggests that the speaker would like to be remembered as a protector of these "innocent" animals, like the hedgehog.
But the last line of the stanza kind of deflates that idea – people might remember him as a hedgehog lover, but they'll also remember that he wasn't able to do much to help them, and now that he's dead and gone, he can't do a thing.
This is awfully sad – the speaker is obviously alive as he is writing, and he's imagining that after he's dead, his life ambitions (to take care of "innocent creatures") will have been for nothing.