A Third Quarter Square between the Capricorn Moon and Aries Sun (18 deg) also involves the Moon's approaching conjunction to Pluto at 21 deg Capricorn (occurring after the square is exact).
This portion of the Moon's Waning (darkening) phase is known as a "crisis in consciousness": there is an awareness that you need to pull away from the past, but the future is unknown and uncomfortable. The past is uncomfortable as well (you can't go back). Hence the crisis.
In the context of this Third Quarter Square, you know something brand new and super unpredictable is brewing, in the guise of the Aries New Moon conjunct Uranus. It's tempting to fall back on old structures and rules (Moon in Capricorn). But those structures are outdated, and Pluto's involvement means they're crumbling away.
Fear not, because this is part of the plan. The Moon's Waning phase is a time of completion. The energy pulls you inward, encouraging you to have a deep think about what's dying - what rigid rules/fears/judgements/traditions/responsibilities are ready to be dismantled and buried? Pluto is enabling your inner work by exposing what's hidden or rotting beneath the structures. Yes, you will have to look closely at some uncomfortable issues.
This is all in preparation for the New Moon's electric new chapter. This New Moon will liberate you, but only if you're not still slogging around in the mud of the past.