This is a brilliant couplet. Mercer references the current state of his relationship using unpleasant, ugly-looking phrasing like “colossal mass.”
Here, for reference, is a colosal mass of garbage

He then states the impetus for this piece-of-shit relationship’s dissipation: his “moat.”

A moat is a man-made body of water surrounding a residence (usually a fucking castle), traditionally to keep off intruders and potential threats. Mercer uses a moat as a metaphor for his standoffish personality and unwillingness to make himself appear vulnerable to most people. This impersonal attitude probably had a very large impact on his relationship.