“This [i.e., First Apology 10] is one of the few passages in which Justin states that God created the world out of unformed matter; cf. 1 Apol. 59, 67. Justin appears to have had no particular theory of the origin and nature of matter but is content to accept Gen 1 as it stands and to see in it no conflict with Middle Platonist teaching. . . . It is also possible that his reference to ‘unformed’ matter may owe something to Wisd. of Sol. 11.17.”
(Leslie William Barnard, Ancient Christian Writers 56.113)
“There is no suggestion in this passage [i.e., First Apology 59] that Justin is criticizing the Platonic doctrine of the eternity of matter . . . . Justin, in fact, mentions formless matter only (cf. Wisd. of Sol. 11.17) in order to emphasize that it was accepted by Moses in Gn 1.1-3, and that, for both Moses and Plato, God had brought the Universe into existence by working on, and changing, formless matter. It is idle to speculate how Justin interpreted Gn 1:1. He is content to accept the text as it stands and to see in it no conflict with the teaching he had received from Middle Platonism (Andresen, 165). . . . A creation ex nihilo does not come within his purview.”
(Ibid., pp. 168-169)
“At 1A[pology] 59.5 Justin says that Plato learnt from Gen. 1:3 (‘God said: ‘let there be light’’) that ‘the whole world came into being by a word of God (λόγῳ θεοῦ) out of the previously existing things mentioned by Moses’—that is, from the formless matter indicated by the invisible and unorganized earth referred to in Gen. 1:2.”
(Dennis Minns & Paul Parvis, Oxford Early Christian Texts 11.62)
(My thanks to Errol Vincent Amey for sharing these sources with me)