danielpugh It’s “cosmetic” in the sense that as long as it doesn’t leak you are good.
When you say “it spins” - is that the hex wrench spinning in the plug, or the whole plug spinning with the wrench?
If the former and it doesn’t leak (FWIW, the brown traces seem to be more sealant/threadlocker than leak), then leave it alone - someone else can drill it out as and when.
If the whole plug is spinning, then you have stripped a thread - if you are lucky, only on the plug, which is easily available as a spare (ACS will send you one, but it will cost an arm and a leg in shipping; these are stainless steel and in the UK: https://www.accu.co.uk/2079-imperial-flat-head-socket-pipe-plugs?page=1 - I think that’s a ⅛″ BSP thread, but check with a caliper before ordering).
If you are not lucky, and the thread on the boiler is gone, then a new boiler is needed.
To remove - I would be very careful with mole grips (as it’s easy to bu99er the thread on the boiler); if you have a drill press, drilling out most of it may be easier.
ETA: if you can insert a knife/screwdriver (or even better a pair of c-clip pliers/thin-nose pliers) under the plug and keep unscrewing, you may be able to remove without drilling - just go easy on the vertical force.