you can combine different conditions in the band maths with
IF x=0 THEN 1 ELSE 0
So first you make a mask with the condition of your algae
Second you make a mask with the clouds
Then you define:
IF mask_cloud == 0 THEN (IF mask_algae == 1 THEN 1 ELSE 0) ELSE 0
This gives you all pixels which are within your algae threshold but outside the cloud areas defined by the filter.