So, this is really just to mop up the details as L. Dutch's answer covers the major problems.
TL;DR: UV is hard to generate and hard to work with. Inefficiencies in the system will make it useless for farming, and dangerous for the user to do anything else other than glow prettily. Sorry about that.
generate UV radiation by consuming cinnabar/mercury ore, converting the mercury into a vapor that interacts with a 300 watt electrical current in their body. This produces UV radiation similarly to how a mercury vapor lamp does.
Mercury vapour lights don't consume that much mercury... it is only lost when the bulb breaks. You won't need a cinnabar rich diet unless you're doing a bad job at sealing your vapour bladders. That is of course up to you as an author, but bear in mind that a species who continuously leak mercury into the environment around them are likely to be unhealthy for other lifeforms in their proximity who aren't similarly immune to mercury poisoning!
So anyway, your species presumably has some gas bladders lined with mercury vapour secreting glands and some kind of electrical discharge mechanism, whatever that looks like. Remember that mecury vapour lights are arc lamps! Immunity to mercury poisoning doesn't imply immunity to UV damage, so your gas bladders will need to be well lined with a UV-protecting membrane if you want to avoid getting glow-in-the-dark sarcomas. They'll also get hot; the process of producing UV isn't perfectly efficient after all, and nor is the conversion of UV to visible light.
Real-world mercury lamps are no more than 10% efficient, I think, so for 300W in you'll get 270W of heat (for comparison, an adult human at rest generates about 100W of heat) so you'll need a decent blood supply to the light organs to stop them cooking their owners. A blood clot could have serious consequences if they don't have effective voluntary control over the light organs... light organ ischemia might result in serious burns or even fires. Beware of senile old folk!
Phosphors in their skin interact with this radiation, causing them to emit visible UV light.
Emitting visible light is kinda at odds to the next set of requirements, by the way.
set up crops by using their UV light to help them grow, which these alien plants thrive in UV light.
Have a read up on the power requirements of grow lights in greenhouses. I'm seeing figures like "30W per square foot". It isn't clear whether luminous efficiency is taken into account, but your glowing peeps might only be able to provide enough light for a few square feet of crops, and that isn't going to produce very much food. It certainly won't provide enough food to offset the calorific cost of having a 300W lightsource in your body.
They harness this radiation, using it to project a UV laser from their eye for self defense against predators and for welding, etc.
UV lasers are super inefficient. Like probably less than 1%. They'll also use a whole different set of chemicals and optics to your UV lights. To produce enough energy to weld is likely to need kilowatts of power, and to do that organically is going to need a pretty hefty appetite, and it'll produce an awful lot of heat which will need specialist radiators or an aquatic habitat to sink. The lasing mediums are often pretty reactive and toxic (lots of lovely halides in there) and would be quite difficult and hazardous to keep in an organic housing.
Focussing and directing UV light precisely enough for metalwork will require some pretty funky looking organic optics, too. I'm not sure how big they'd need to be, but you might start needing a head like Giger's alien.
As a defense... well, you don't need many watts to blind someone, but if you were intending to zap predators with deadly eye beams you'll run straight back into that thermal inefficiency problem with kilowatt power draws and heat outputs. Really, you'd be more effective if you used all that energy to drive massive muscles and threw rocks instead!