Regulation 422.3.1 states "Unless otherwise recommended by the manufacturer, a small spotlight or projector shall be installed at the following minimum distance from combustible materials:
(i) Rating up to 100 W 0.5 m"
This is repeated with regulation 422.4.2 both which require "a luminaire shall be kept at an adequate distance from combustible materials."
Just a guess, but I think it is probably condensation causing a mold to form.
As Eric says it is probably too close to the beam. Is it possible to fit some sort of heat proof reflector, the type you see in hand held inspection lamps.
Besides the regulations concerning distances between lights and combustible materials being complied to!
The stain is likely to be blushing, this is a moisture issue, could be down to leak, condensation or trapped moisture in timber if it has been sealed.
The affect could be being increased be the heat of the lamp.
If the wood is exposed and not sealed an old housewives cure, is a mixture of olive oil and vinegar to remove it, but whether the heat of the lamp will create another problem I could not say, it may be completely coincidental that the stain is in the same location as the light fitting.
The landlord has a legal responsibility and a duty of care to their tenants to ensure the property is safe of danger or potential hazards.
They must prove in some way, that they have taken the correct actions to prevent any hazards or potential hazards. Unfortunately unlike gas, the electrical part gets overlooked and the powers that be, do not actually stipulate the best method or proof that should be performed/evidence of to comply to this requirement. I would suggest that only an "electrical installation conditional report" would be really suitable. And PAT testing of any appliances left under the agreement.