Use solar cell to power and communicate with chip in IR
Both arc welder and a plasma cutter require fairly thick power cables and gas hoses. Compared to them the stiffness of the power + communication cable for MCU is practically non existing.

The second thought is that arc welder produces very strong EMI, which will affect MCU and communications without proper shielding (adding more weight to the movable part)

So, my first suggestion is to consider using single STP cable for both power and communication. Note, that the cable should not go straight between those two points 10 cm apart, it should loop sufficiently to support possible need for equipment separation, maintenance or introducing additional nodes into communication. Which coincidentally makes stiffness of the cable less important.

A second suggestion can be used if arc welder is a movable part and if it uses AC current. In this case you can put a core with a coil (current transformer) around the welder cable and leech power right at the end, sort of point-of-use power supply.

The third suggestion does not require AC welding current but it does depend on your definition of "slightly movable". If you are talking about several inches then you can use toroidal transformer with air gap between the core and one of the coils, big enough to accommodate the movement, as in the picture below.



Also, if you still want to explore solar cell solution, note that you don't need light emitting part at all. The brightness of an ark and plasma is way more than you can achieve with any kind of LED.