a photo would really help us to see what the problem may be.

is my soil mix ok? should I use the same soil or add more grit in it?

why do those leaves get softer?

Don't know why your leaves are soft. Is that the entire plant? How big was the pot? Did it have a hole in it? I ask as all these aspects matter & are telling in trying to troubleshoot a plant problem.

Pictures of the whole thing, entire plant in pot w/ soil are much more telling than a leaf here, the soil there, kinda frustrating.

You don't seem to know this is a tropical plant, so it needs water FAR more frequently than once a month. That soil is not good for this (sorry to say), looks like maybe dirt from your yard?

I'd give this Cactus & Succulent mix, combined w/Perlite; 50/50 of each. If you can get small bark that would be good too, 1/3 of each C&S mix, Perlite & Bark.

These plants naturally grow hanging from trees (that's what it means to be an Epiphyte), not in soil, so they need super fast draining mix & SHADE, not direct sun. I'd also use a smallish pot, not much larger than the plant.

May I pls. ask what part of the world are you speaking from? Somewhere in Asia?

Why are you so concerned that the leaf is soft? Are the others not?

Agree with Karen that a photo of it planted will tell us more, but that seems impractical now that is it unpotted. You can get some small bark, orchid bark, mix into soil, mostly bark and grit, and yes, do water more that once a month, unless it is getting rain. Here's a site that has lots of epi info, since it's not really like growing most other succulents.

www.epiphyllums.org/FAQs

They both look to be planted a little deep, the soil surface should be closer to the top of the pot. And as stated before, I can see the soil is to peaty, need much more bark and grit. Happy growing!

I’m wondering why you’ve started a new thread about the same plants you asked about on your dragon fruit posts? https://www.gardenweb.com/discussions/5476332/should-i-prune-my-dragon-fruit-plant#n=27 People may not know the back story.

Btw, the medium is not the soil from a British garden! And the OP lives in a fiat, so has no ‘yard’.

this thread is about my epiphyllum plant. that one is about my dragon fruit plant(dragon fruit plant is not an epiphyllum variety). It is about different plant with different problem. So, I shoudn't post them on the same thread right? I just use the same photo cause the only recent photos I took of my epi are those next to my dragon fruit plant.

It wasn’t clear to me that you were referring to a different plant because you showed the same photographs.