The Ur-Quan killed the Taalo under mind control by the Dnyarri
No they didn't. After the Dnyarri were defeated and the Ur-Quan started executing their Eternal doctrine, they arrived at the Taalo homeworld. Then, the Taalo said the magic words "Hold, what you are doing to us is wrong! Why do you do this thing?"
This question had such a great impact on the Ur-Quan that they had to answer it thoroughly. And while they were explaining themselves to the Taalo, the Kohr-Ah arrived and a battle with the Ur-Quan followed. During that battle the Taalo escaped and are now "playing time tricks" as the Orz mentioned.
"We will explain to you about the Dnyarri our slavemasters
the Taalo, our only friends... whom we exterminated
and our reasons why we cleanse the galaxy of all other sentient life."
"Who were the Taalo?
Of all the species we have met, only the Taalo did not trigger our instinctive territoriality.
They were the only people we could stand with, or talk to, without the hunter inside us screaming
`Kill the interloper! Rip out its life!'
We believe that the same factors that made the Taalo non-threatening to us
their unusual rock-like biology
also gave the Taalo natural immunity to the Dnyarri's psychic compulsion.
The Dnyarri would not permit this.
After they had conquered all six of the other races in the Milieu
they commanded us to attack the Taalo and we did.
The Taalo would not fight back... they died.
I am familiar with the dialogue of the Orz hinting that some of the Taalo still might be alive in some other *time* or dimension or whatever, however the race you are referring to who uttered the famous words "Hold! What you are doing is wrong, why do you do this thing?" Was the Melnorme.
Note that all of this also occurs before the Ur-Quan defeated the Dnyarri. After all it would make zero sense for the Ur-Quan to willigly kill their own allies, "the only race we called friend" in their own words, no?
EDIT: Mixed up two races whilst crusading against misinformation. DERP.