Great advice thus far. Big water changes are important for removing any significant amount of pollutants. Most fish will not starve themselves to death and certainly not a walking catfish so have patience, it may still be settling in 

Hi all I’ve just recently bought a 12” -14” inch walking catfish from a friend who just couldn’t find time to keep anymore. Recently moved his tank and changed his features and he’s loving it. Only struggle I’m having at the moment is he’s only interested in bloodworm blocks and I don’t know what else to feed him. Concerned to if I’m not feeding enough or over feeding but don’t want to contaminate the water with waste food he’s not supposed to have or wants. Any help with what people have fed there’s before or know is really appreciated! No luck searching online
Just moving away from feed.
A few other things you may pick up about the fish.
The marbled Clarias can be expected to get to 18” ( minimum) in a tank and at that size will be a GREAT escape artist. Ensure you have a tight fitting, heavy lid, otherwise you will be finding him on the floor.
The other thing you may learn is that Clarias love to re-decorate and can be expected to pile the gravel where they want it. As such, an under gravel filter may not work so well. (Can be tried but you may end up reverting back to a mechanical). The heater should be off the bottom also as the cat will often lay against it if it is in too handy a place and the fish could burn itself.
For info on the fish itself, planetcatfish.com
Mine eats api bottom feeder pellets and occasionally blood wormsHi all I’ve just recently bought a 12” -14” inch walking catfish from a friend who just couldn’t find time to keep anymore. Recently moved his tank and changed his features and he’s loving it. Only struggle I’m having at the moment is he’s only interested in bloodworm blocks and I don’t know what else to feed him. Concerned to if I’m not feeding enough or over feeding but don’t want to contaminate the water with waste food he’s not supposed to have or wants. Any help with what people have fed there’s before or know is really appreciated! No luck searching online
Don’t think you need to worry about what to feed. When he gets hungry I will be extremely surprised if he doesn’t eat just about anything. If it’s usual feed for fish of any kind, he/she will eat it. Try whitefish, sprat or tilapia or mussel meat, but also decent sinking pellets.
As you note though, need to keep an eye on water quality as Clarias are eating machines and really good at begging for food.