No problem if its leaking from the screw its a sign the valve is starting to fail, you will probably find some lime scale has hardened around the minor washer you likely over compressed when trying to shut the water off.
It will probably be fine for many months, but start leaking over time or every time the toilet is used and the pressure in the pipe changes.
If you want to change it yourself (rather than notify the landlord) should it start leaking again, it would take a novice 30 minutes at most, have a look at the video here, it gives you the general idea of what is required.
To do this repair you ideally need two mole grips \ large pliers one holding the valve so it doesnt move the other unscrewing the nut, you can pick a pair up on ebay for £6. You will need a replacement valve which is about £4 from screwfix here, make sure you get the right size 15mm, I would also buy some PTFE tape which is about 50p from screwfix here. So all in all its going to cost about £10 including the tools.
The PTFE is only require if when you turn the water back on the valve starts to leak you wrap it around the compression two times and it helps with creating a water tight seal, the end of the video here probably explains it better than I am.
Please let me know if you need anything explained, appreciate if you can rate the service received so if not so I can close the case.