Yabby shell rot?

I really need help with this as I came back from a 2 week holiday to find my male yabby like this! The person who was looking after my tanks for me says they never noticed anything but I don't see how that's possible. I think it may be shell rot but I'm not sure if it's too far gone, let alone how to treat it properly.
Please help!

Tank size: 360L
pH: 7
ammonia: 0
nitrite: 0
nitrate: 0
kH: 6
gH: 8
tank temp:

Fish Symptoms:
No shell on a section of his left claw, the red marking on the right claw is tranluscent, next to no appetite, spends most of the day just gently waving his legs.

Volume and Frequency of water changes: 25% every 2 weeks

Chemical Additives or Media in your tank: Fine sand with some medium sized smooth pebbles. I add Shrimp Mineral GH/KH+ (https://www.amazon.co.uk/dp/B06Y5RXML6/?tag=) with each water change.

Tank inhabitants: Mollies, Platys, Swordtails, x-ray tetra, a flying fox fish, a plec, Bronze corys, Bamboo shrimp, Cherry shrimp, and Nerite snails.

Recent additions to your tank (living or decoration): None

Exposure to chemicals: Only the Shrimp Mineral GH/KH+ and tap safe

Is it meant to have the cream patch on the undamaged claw?

I can understand the crayfish being upset. Its claw has been busted open and bits of flesh removed. It must be bloody painful.

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Wipe the inside of the glass down with a clean fish sponge.

Do is a 75% water change and gravel clean the substrate every day for a couple of weeks. Then do a 75% water change once a week.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

Clean the filter if it hasn't been done in the last 2 weeks. Wash filter media/ materials in a bucket of tank water and re-use them.

Increase aeration/ surface turbulence to maximise oxygen levels.

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Add some salt.
You can add rock salt (often sold as aquarium salt), sea salt or swimming pool salt to the aquarium at the dose rate of 2 heaped tablespoon per 20 litres of water.

If you only have livebearers (guppies, platies, swordtails, mollies), goldfish or rainbowfish in the tank you can double that dose rate, so you would add 4 heaped tablespoons per 20 litres.

Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.

The salt will not affect the beneficial filter bacteria but the higher dose rate will affect some plants. The lower dose rate will not affect plants.

After you use salt and the fish have recovered, you do a 10% water change each day for a week using only fresh water that has been dechlorinated. Then do a 20% water change each day for a week. Then you can do bigger water changes after that. This dilutes the salt out of the tank slowly so it doesn't harm the fish.

If you do water changes while using salt, you need to treat the new water with salt before adding it to the tank. This will keep the salt level stable in the tank and minimise stress on the fish.

also be aware that the crayfish can eat swordtail and small fish, i once have 2 crayfish and it eat all my guppy and swordtail.

Thanks for the advice guys
Updates!
Bob seems to have perked up in the last day or so! He's still not as active as he was but he's started to use his claw again. Also I think his shell may have started to grow back? The exposed fleshy area is now very smooth like he has a thin layer of shell over it now, does this sound about right with shell re-growth?
If I can get a clear picture of his claw I'll post it on here :3
Again thank you so much!

Barry Tetra said:

also be aware that the crayfish can eat swordtail and small fish, i once have 2 crayfish and it eat all my guppy and swordtail.

Click to expand...

Yeah he's eaten a fair amount of fish since we got him! But we have him on a diet of shrimp pellets, algea wafers and raw king prawns so he only ever goes after the old/sick fish (even then not all the time). Strangely enough though he's never touched my plec or corys!

You shouldn't feed raw prawns to crayfish because they can catch diseases from the prawns. If you want to use prawns as food for tanks containing crayfish or shrimp, cook the prawn first, then feed it.

Need a picture of his claw now to check that it is healing and not covered in fungus.

Really? I was told raw king prawn was great for him I'll try and get a picture as soon as he stops digging.

There is a number of diseases that affect shrimp and crayfish and they are readily transmitted by eating raw contaminated shrimp or crayfish flesh.

I feel so aweful that I didn't know raw shrimp was dangerous, I called the shop that we got him from since they still sell crayfish and they were adament that raw is fine He's had raw shrimp since we got him 2 years ago so I can only imagine how lucky we are.
Anyways I was finally able to get some decent picture of his claw! The affected area hasn't spead and the flesh is now smooth (is that good?) he's also using his claw to help him move around again so does that mean he's in less pain now?

It looks better and if the crayfish is using it again, that is a good sign.