Stop your pump keep your prime. Turn control unit to off. and let it drain down to the pump or the leak and let gravity do its thing unless you already know the manufacturer. Go to Menards or order a replacement valve. Looks like someone loosened it to the point of leaking or the valve's head cover for a lack of a better term should have four screws. Valve should be about 15 bucks and i'd turkey baste that water out or ensure your power wire/control unit cannot and willnot turn on to be safe. Just replace the top of the valve, take the 4 screws out lift up take the top of the new one, you should be able to see whether it was the plunger or a seal that just went bad or what. If it's at the pipe which isn;t usually the case unless your foot stepped down in there without the cap on the valve covering. usually changing the top of rain birds and hortons clears up the bad seals and the bad plunger without you having to dig out or get under it and undo both sides of the pvc or funny pipe. The knob with the wires should have arrows pointing on/off directionally righty tighty on mine id assume yours is as well. tighten that pup up before switching it to see if it'll seal. Should be a slot for a big flat head screwdrivers or sprinkler valve tool/key if yours came with one where you can open and shut the valve manually. it could be seized and leaking out of sides and seals under immense pressure or it's just a basic maintenance time + wear + climate +vibration = leaks. Try replacing as much of it as you can with just using the top half of the new valve. it's a trick that usually allows you to change most of it and fix that most of the time otherwise drain it down somehow and id where the leaks coming from... inlet outlet, plunger, crack, breakage in plastic valve or pvc, if you have an underground siphonless or special valve you mau just start by replacing but that holes only a foot or so deep and usually dirt floored and not a big deal to work on. once you have part i'd say like an hour after safety prep ensuring systems off and doing any water removeable if necessary otherwise you call leave it for a few days and let it soak back into the ground perhaps then work on it too. Good luck.
