This. Many people don't seem to notice this for some reason, but just because Phos doesn't outright tell Aechmea "that smells like fucking bullshit you evil manipulative hateful fuck", it doesn't mean that he isn't thinking it.
A lot of what Phos is thinking is shown through his body language and facial expressions.
Phos 100% doesn't trust Aechmea, but also Phos has absolutely no power over Aechmea, which was made clear to Phos during their first meeting when Aechmea teleported behind him and cut off all his limbs within half a second.
Not to mention that the moonies can at any time destroy the entire gem society and kill all the gems. They only choose not to because they're trying to mentally torture Sensei by letting him bond with each of the gems before stealing them away.
So actually it doesn't even matter if there are gem-hostages on the moon or not; Phos' entire world and family is a hostage at all times if Phos doesn't do what they want.
And of course Phos doesn't care about his own safety, but Phos also knows that if he dies nobody will pick up the mantle for what he's doing, so he has to try his best to sort out the situation himself for the sake of the ones he loves.
To make a long story short, Phos has to be super fucking catious of what he says with Aechmea because Archmea can easily nuke Phos if Phos proves to be inconvenient.Of course, Aechmea needs Phos because of the pearl eye but Phos doesn't know that.