Modal verbs can be used in a variety of different forms. Study the examples below.
I could swim at the beach.
I could be swimming at the beach right now.
I could have swum at the beach yesterday.
Modal Perfect Continuous
I could have been swimming at the beach instead of working in the office.
Passive Modal Simple
The room should be cleaned once a day.
Passive Modal Continuous
The room should be being cleaned now.
Passive Modal Perfect
The room should have been cleaned yesterday.
Passive Modal Perfect Continuous
The room should have been being cleaned but nobody was there. (Rare form)