Molt Sac? Is he getting ready to molt?

[Xenpurgisn@cht]

My largest crab odin I believe is showing molting signs again, he's digging around ( but covering himself ) and I saw him change shells yesterday I believe I saw a molt sac, it was a large blackish bulge at the very front of his tail, but under him to the point where I couldn't see it- I'm not sure if it was a molt sac as I've thought molt sacs was always visible. Odin himself doesn't really dig until he's showing signs of molting, but cases of them wanting to dig for fun still happens regardless. Does it sounds like he's getting ready to molt? He's a large, and the last time he molted was in may which actually in the shortest timeframe between molts- I've had him for 2 years now and on average, he's usually waited 8ish-12 months to molt.

Is what im describing a molt sac, and does it sound like he's getting ready to molt?