As confirmed by Christopher Tolkien's commentary, Angrod is referring to Caranthir's reaction to when he delivered Thingol's welcome message sixty years prior.
Angrod son of Finarfin was the first of the Exiles to come to Menegroth, as messenger of his brother Finrod, and he spoke long with the King, telling him of the deeds of the Noldor in the north, and of their numbers, and of the ordering of their force; but being true, and wisehearted, and thinking all griefs now forgiven, he spoke no word concerning the kinslaying, nor of the manner of the exile of the Noldor and the oath of Fëanor. King Thingol hearkened to the words of Angrod; and ere he went he said to him: 'Thus shall you speak for me to those that sent you. In Hithlum the Noldor have leave to dwell, and in the highlands of Dorthonion, and in the lands east of Doriath that are empty and wild; but elsewhere there are many of my people, and I would not have them restrained of their freedom, still less ousted from their homes. Beware therefore how you princes of the West bear yourselves; for I am the Lord of Beleriand, and all who seek to dwell there shall hear my word. Into Doriath none shall come to abide but only such as I call as guests, or who seek me in great need.'
Now the lords of the Noldor held council in Mithrim, and thither came Angrod out of Doriath, bearing the message of King Thingol. Cold seemed its welcome to the Noldor, and the sons of Fëanor were angered at the words; but Maedhros laughed, saying: 'A king is he that can hold his own, or else his title is vain. Thingol does but grant us lands where his power does not run. Indeed Doriath alone would be his realm this day, but for the coming of the Noldor. Therefore in Doriath let him reign, and be glad that he has the sons of Finwë for his neighbours, not the Orcs of Morgoth that we found. Elsewhere it shall go as seems good to us."
But Caranthir, who loved not the sons of Finarfin, and was the harshest of the brothers and the most quick to anger, cried aloud: 'Yea more! Let not the sons of Finarfin run hither and thither with their tales to this Dark Elf in his caves! Who made them our spokesmen to deal with him? And though they be come indeed to Beleriand, let them not so swiftly forget that their father is a lord of the Noldor, though their mother be of other kin.'
The Silmarillion chapter 13 - "Of the Return of the Noldor", adapted from The Grey Annals §63-67 - Year 7
Now years later, Angrod who had previously held himself back from telling Thingol about the kinslaying remembers how ungrateful Caranthir was of it, and decides that it isn't worth holding back anymore now that he himself is being accused of it.
'I marvel at you, son of Eärwen,' said Thingol, 'that you would come to the board of your kinsman thus red-handed from the slaying of your mothers kin, and yet say naught in defence, nor yet seek any pardon!'
Then Finrod was greatly troubled, but he was silent, for he could not defend himself, save by bringing charges against the other princes of the Noldor; and that he was loath to do before Thingol. But in Angrod's heart the memory of the words of Caranthir welled up again in bitterness, and he cried: 'Lord, I know not what lies you have heard, nor whence; but we came not red-handed. Guiltless we came forth, save maybe of folly, to listen to the words of fell Fëanor, and become as if besotted with wine, and as briefly. No evil did we do on our road, but suffered ourselves great wrong; and forgave it. For this we are named tale-bearers to you and treasonable to the Noldor: untruly as you know, for we have of our loyalty been silent before you, and thus earned your anger. But now these charges are no longer to be borne, and the truth yon shall know.'
The Silmarillion chapter 15 - "Of the Noldor in Beleriand", adapted from The Grey Annals §104 - Year 67
Both of these passages derive (with some editorial changes) from the Grey Annals, which has been published in The War of the Jewels. Caranthir's harsh words are said in year 7 in §GA67. Angrod remembers the words in year 67 in GA§104. In Christopher Tolkien's commentary, when describing an earlier draft of The Grey Annals, confirms that it was a reference to the first passage.
This passage was bracketed, and later in the draft the bitterness of the memory of Cranthir's words of sixty years before appears in Angrod's mouth, as in GA (§104).
The History of Middle-earth volume XI - The War of the Jewels - Commentary to The Grey Annals