Board Thread:Questions and Answers/@comment-222.154.31.233-20130121220943/@comment-6024139-20130124171317

If you mean the bar highlighted on the image it's sludge. You can pump it out with a waterproof pipe.

You can read more here:

http://www.minecraftforum.net/topic/1629898-146-powercrystals-mods-powerconverters2-is-here-mfr-and-more/