These are space-faring aliens we are talking about. Such creatures would likely use up every resource - and forward energy and information to the front, using lasers, with relays if necessary. There would be practically nothing left behind at all. The idea that they would be unable to utilise some kinds of planetary or solar resource - because they are too small and insignificant - does not seem remotely plausible to me.

Remember that these are advanced aliens we are talking about. They will be able to do practically anything.