Let's fix the terminology a little. You have not SLAMed your pool. You have started the SLAM process but you are not done.
Yes, you can add CYA during this SLAM but I am going to withdraw that suggestion now that I know you get so little sunlight.
So, you are in the middle of the SLAM process and you are replenishing your FC back up to 12 ppm at least twice daily, correct.
Vacuum that dead algae off the pool floor, backwash as necessary, and keep running your pump 24/7. You should see less and less of that dead algae on your pool floor and your pool should get clearer as you progress......it will be a daily improvement if you keep your FC up.