Don't worry mate... nothing serious!!!
From what you describe it appears to be a cam-cover leak. Since you have a 155 this should be an Aluminium one with the writing painted in red, right?
Ok, these things are very fragile since Alfa Romeo didn't do their homework (once again!!!); if you overtighten the cover by even a little bit you will crack the edges of it enough to promote a leak. These covers are around £160 new so what I would suggest is to buy a 2nd hand 156 black plastic cam-cover (comes with gasket but make sure to get the screws with it since these are different).
The only tricky bit is to find new hoses for the oil recirculation pipes to fit to the new cover since these are in different positions to the old one. My advice is to get hold of 156 hoses (brand new these are only £10 together I think) and modify away. I've made my own in a different way but that's because I work for a motor company so I had an entire parts-bin to play with
If you want to know more about this mod you should ask HENRY YORKE in this forum since he has done the same to his 155. Your bracket where the spark-plug coil packs are fitted to will fit on this but expect a slight clash with one of the screws on the cover (don't worry about this, ok?).
If you're bothered about the looks you can get hold of the beauty cover to fit on top of this which will make your engine bay look MINT!!!
This cover though requires slight modifying at the rear Rh side due to the position of the oil recirculation pipes (these will clash with the cover). Get hold of a Stanley knife and cut away!!! use some sand-paper to round/smooth off everything since you don't want to cut these pipes or they'll mess up all your hard work.
All in all this mod should not cost you more than £50 all in if you're prepared to do some of the work yourself.