I did a milk chocolate ganache to top my coffee cupcakes and it looks 'ok' under normal light but I'm a little worried.

I understand that sometimes the ganache can go wrong by 'separating' into an oily muck.

Mine hasn't quite separated into something like that, but under direct light, instead of an even glistening top, it looks more like there's a very thin film of oil.

I'm not sure if this is from the butter in my cake or if the ganache has started separating.

I've circled it to show you what I'm talking about.

Were you using ganache that you had just made or was it made previously and melted down to use?  When using ganache that has been made on a previous day, heating it to liquify it can cause it to "break", and you usually can see the separation. When ganache "breaks" you can use a small amount of scalded heavy cream to pull it back together.  To me, your cupcakes look fine, and I don't think I would worry too much about them.  I certainly wouldn't re-do them.  :)

It was a freshly made batch. I think it may have just been the weather, it's warm enough that even my cakes wound up a little overbaked. I let them sit in an air conditioned room for a couple more hours and the film seems to have vanished.