I read that Space contains a "grease" like substance that will cover any spacecraft during a long flight. My question is how will it be removed because I assume that you can't just take a bucket of water out in space to wash it off?
I don't know of anything which is a perfect match to your description, but I can think of two possibilities which are close:
First, space weathering is an important and more-or-less continuous process. Basically, impacts by micrometeorites, cosmic rays, solar ultraviolet, and the solar wind slowly messes with any surface exposed to space. The effect mainly darkens it -- this is why the Moon's surface has approximately the color and darkness of charcoal briquettes. It's a slow process -- I believe that the beginning effects have been seen on spacecraft parts exposed to space for years, but the best evidence comes from looking at how the material turned up by meteorite strikes on the Moon slowly darkens.
The other possibility is that there is a small amount of "dust" (that's what astronomers call any fine particulates in space) which is complex, carbon-rich organic gunk, a bit like the tars you get when you combust wood or coal in an oxygen-poor environment and a bit like the soot from a smoky candle flame. Even in the densest regions this is very diffuse, but, given sufficient time, it might well form a coating on an exposed surface.
Neither process occurs fast enough to be a problem on a human timescale. (Sadly for the many stories which drew parallels between space navies and the Royal Navy in the Napoleonic Wars, there will never be a need for the crew to holystone the decks in space.)
A definition of grease is that it's carbon based. Carbon provides a junction to make billions of compounds with the solubility and flexibility of water and the diversity of minerals, so it can make some kinds of flowing minerals, grease, syrup, solvent, tar, and every type of liquid variability imaginable.
The recent research states that:
Until now there has been uncertainty over how much carbon is drifting between the stars. About half is expected to be found in its pure form. The rest is chemically bound with hydrogen in either a grease-like form, known as aliphatic carbon, or as a gaseous version of naphthalene, the main chemical component of mothballs.
The degree to which space-naphtalene condenses onto warm/cold spacecraft is only theoretical, and hasn't been noticed and measured on previous space programs as far as a google search can find.