What should you do if you a see a red and white light on a vessel at night?

Without seeing the actual configuration there isn't enough information to make a decision. Am I seeing the port side aspect of of the vessel? A port side running light at 112.5 degrees and a masthead light at 225 degrees? Where are the lights in relation to each other?

Also with the information you have given it could be a a fishing vessel other than trawling, underway, not making way (red over white). OR it could be a pilot boat underway, engaged in pilotage duty (white over red).