!!!This deck will not be super competitive. I'm guessing you're aware of that, but I wanted to point it out just in case. Mill decks in general have a tough time in Modern, and this one in particular relies heavily on a combo that will be difficult to use.

About the combo

The biggest weakness is that it's going to be really difficult to pull off a combo that requires four cards. Right now there is only one way your combo works:

[[Hedron Crab]] + [[Walking Atlas]] + [[Retreat to Coralhelm]] + [[Dimir Aqueduct]] = Win

If you are missing any of those cards, the combo can't work.

You could add a few other things to diversify the combo a bit more:

[[Cloudstone Curio]] will enable your combo to work with any land instead of just [[Dimir Aqueduct]]. If the lands come into play untapped, you can even generate infinite mana (for instance, you could just bounce two Islands back and forth, tapping each one for mana before sending it back). And if you generate infinite mana, you can use [[Mind Grind]] as your source of mill instead of [[Hedron Crab]]. And if you can generate mana while bouncing lands, you could use something like [[Freed from the Real]] to untap [[Walking Atlas]] instead of [[Retreat to Coralhelm]]. Lastly, [[Amulet of Vigor]] would let you untap and use [[Dimir Aqueduct]] before it returns to your hand, also allowing for infinite mana.

If you swap in those cards, you go from one possible instant-win combo to eight:

[[Hedron Crab]] + [[Walking Atlas]] + [[Retreat to Coralhelm]] + [[Dimir Aqueduct]] (The original)

[[Hedron Crab]] + [[Walking Atlas]] + [[Retreat to Coralhelm]] + [[Cloudstone Curio]] + any land

[[Hedron Crab]] + [[Walking Atlas]] + [[Freed from the Real]] + [[Cloudstone Curio]] + any land that enters the battlefield untapped

[[Hedron Crab]] + [[Walking Atlas]] + [[Freed from the Real]] + [[Dimir Aqueduct]] + [[Amulet of Vigor]]

[[Mind Grind]] + [[Walking Atlas]] + [[Retreat to Coralhelm]] + [[Cloudstone Curio]] + any land that enters the battlefield untapped

[[Mind Grind]] + [[Walking Atlas]] + [[Retreat to Coralhelm]] + [[Dimir Aqueduct]] + [[Amulet of Vigor]]

[[Mind Grind]] + [[Walking Atlas]] + [[Freed from the Real]] + [[Cloudstone Curio]] + any land that enters the battlefield untapped

[[Mind Grind]] + [[Walking Atlas]] + [[Freed from the Real]] + [[Dimir Aqueduct]] + [[Amulet of Vigor]]

Note that it's still difficult to assemble the combos even when there are many possibilities. Also, all of the combos rely on [[Walking Atlas]], so you may want a way to dig for it.

The deck in general

Since you don't have any red and can't use the Entering half of [[Breaking // Entering]], [[Glimpse the Unthinkable]] is a strictly better replacement.

If you're hoping to win off the combo, you can cut back on the mill cards that aren't part of the combo. Once the combo starts, anything else that has happened before is irrelevant. So if you want to make it a pure combo deck, you would want to cut [[Breaking // Entering]] / [[Glimpse the Unthinkable]] and [[Mind Funeral]], and instead replace them with cards that help you get to the combo faster. If you want to use a general mill deck with the combo being just a fun possibility (but not the entire game plan), keep the other mill cards in.