Unfortunately yeah that is duct tape across it. The fins are very uneven and the airflow wasn't as good without it. I compared the thermal with and without using my flir (I do understand this isn't the most accurate method). After 20 minutes there was a 10 degree (5C) difference in favor of the tape.Beautiful color!
Is it a duct tape on a heatsink? Do you really need it? It covers quite a lot of surface and reduced heat exchange.
Also consider blowing area across the laser base (not a long) to reduce flowing air influence on the beam.
I would say it is fibrous gypsum ... ?
Thanks for the pics ---- !