This looks great! Now, the tough part -- detecting enough light to get a spectrum. If you let the sample settle for a few days, does it look more clear, less muddy?

I'm not sure about attenuating the laser, but it definitely excites the petroleum in the sample, causing it to fluoresce in red or orange. Check out this test we did where we were actually able to detect the fluorescing oil in a sample, with a very similar setup to you:

http://publiclab.org/notes/warren/05-14-2013/laser-oil-fluorescence-proof-of-concept

Perhaps we need a better camera in the spectrometer, though, like Chris F's Ebert. Another approach might be to use some filter material which blocks green light, so that we only see the fluorescing light, not the source laser light... that way we can get a clearer signal without the green blowing out the exposure.