white scum on top of robust porter

I have a robust porter in secondary and theres a white chalky scum on top of it. is this bad, or will it be ok to keg.

Not really any way to tell on this side of the screen.  If you see any stringy things hanging from the surface then you may have an infection.  Pull the ferm. lock off and take a smell.  If it smells ok then take a taste.  If it tastes ok then you probably have nothing to worry about. 

There is something that is called pellicle, it is one possibility for what you have there.  If that is the case, it is nothing to worry about.  As DaveinPA wrote, smell it, taste it (most important)...if you don't immediately want to spit it out and vomit, then you should be ok. 

You need to provide pics or more info to really get some good feedback for this type of thing.

kegged it today. smelled it, tasted it, all was ok. after forced carbing it, it was actually ok. used london esb yeast so its a little dry. but not bad. certainly drinkable.