No idea how it got there but you should be able to suck out most of the contents then remove reservoir to clean after detaching pipe immediately underneath. This short vertical pipe to master cylinder should retain fluid so on re-assembling DOT 4 fluid could be added without need to bleed.
Maybe a flush through after that would be wise though if there is a bleed nipple, although I believe your GDI Plus 4 has a concentric slave cylinder as with 3.7 V6 so this may not be possible.
