Can you provide us with ammonia levels? Although, I don't believe it's anything to do with water as your rasboras would also be affected?
I'm thinking it could be something molly specific, since your rasboras are unaffected by it?
(I'm seriously questioning myself lol)
Then again, we still need your water parameters. Your tank may not be cycled or at least be properly cycled, or a factor could have crashed it. Cloudy water usually is called new tank syndrome I believe, and would make sense as yours isn't 2 months old. It's just bacterial bloom, not dangerous but a sign of a new unstable/uncycled tank.
Are you able to get a proper accurate test kit? API Master Test is a super good one. Hopefully you can get one off Amazon with a really swift delivery, or at a store.
Can you provide us with your overall water hardness (in ppm)? Does your tap water have any ammonia/nitrites in it? Do you use an aquarium specific dechlorinator? If I recall correctly, for livebearers carbonate hardness also must be quite high. Or is that malawi cichlids? I might be confusing myself lol.
Though I really am sorry about your loss, I'm still sad about my guppy fry that jumped out along with passing of two of my shrimps
It's never pleasurable to lose your pets, even if it's "just a fish". Best we can do is just keep picking at issues until we find a cause, it's impossible to tell what's wrong with a fish unless it's something we can see with our naked eyes.
I really hope we'll be able to find the issue together, I really do.