I'm pretty sure that stuff is just a residual substance from one of the chemicals used inside the arc tube. I've seen that in both 400W and 600W conversion bulbs that I used in the past, and all seemed to operate just fine for me. I've also used both Sunmaster and Hortilux conversion bulbs, and they both had that same residue. Nothing to worry about, as long as it's lighting up, you're good to go! :thumbsup: