According to the Oxford Latin Dictionary, the third-declension adjective informis means:
1 Having no definite or regular shape, formless, featureless, or sim. b (of an abstract idea, institution, or sim.) not having a material form. c bodiless, disembodied.
Although no related noun is attested in Classical Latin, it's simple enough to derive one on the model of, e.g., brevis > brevitas. So, if you're open to using a neologism, the noun informitas (genitive singular informitatis) would be one possibility.
Also, note that Forma Tempii doesn't mean 'the Form of Time'; the phrase should be Forma Temporis, because tempus is a third-declension noun.
Update: As sumelic has noted, informitas actually is attested, though in Late Latin, and means 'unshapeliness, ugliness, deformity' (according to Lewis & Short, which I should have checked in addition to OLD to begin with). So, yes, my suggestion of Forma Informitatis might be misleading.
For alternatives, perhaps one can glean something from, e.g., book 1, lines 5–9 of Ovid's Metamorphoses, since that passage describes pre-creation formlessness:
ante mare et terras et quod tegit omnia caelum
unus erat toto naturae vultus in orbe,
quem dixere chaos: rudis indigestaque moles
nec quicquam nisi pondus iners congestaque eodem
non bene iunctarum discordia semina rerum.
Line 6–7 say that nature had one appearance in the whole world, and that this was called 'chaos.' L&S gives as definition II of chaos 'The confused, formless, primitive mass out of which the universe was made,' and OLD gives 'The formless state of primordial matter or the period of this state.' So maybe Forma Chai would work. The problem there is that it could be taken as chaos in the modern sense of the word. Maybe something can be done with the phrase rudis indigestaque moles instead.
Or, there's bound to be something in Lucretius.
Update 2: I note that Apuleius (De Platone et eius dogmate 1.5) uses the adjective informis specifically in the context of Plato's forms:
initia rerum esse tria arbitratur Plato: deum et materiam inabsolutam, informem, nulla specie nec qualitatis significatione distinctam, rerumque formas, quas ἰδέας idem uocat.
So, if one goes by classical meanings, I still think there's some justification for the noun informitas.