think you got your solution right there .. transplant it asap... do that cup even have some nice drain holes in the bottom ?
weed need a light airy soil and good drain .. and a pot size that allow it to grow ..
if your concerned about cal/Mg there is alot of products you can buy for that or just use Epsom salt and lime .. both are stuff I like to mix in to my soil to start with ..
btw
to me it mostly look like it run out of fresh soil/nutriens/Nitrogen ..
the light green leaf`s edges look like its low on food .. dunno .. dont know how strong happy frog soil is on nuts ?