Yep, shock shock shock. Run the pump 24x7 while shocking. do your best to brush the areas like the niche if you can until the algae dead and gone. The high CYA combined with not much flow in behind the light into the niche area are probably the cause of the problems. As long as water is able to flow into the drains goo is coming out of, I am pretty sure shocking with a manageable CYA level will take care of those. Make sure to manage your pH, since it was already over 8 and your water has 450 CH. Scaling is likely in that combination.

I'm not sure if it's safe (or worthwhile) to try and get something physically into the algae filled drains to help things along. Hopefully someone else will comment.

Your CC will probably climb back up some as the chlorine does its job.