Appears to be carbon of some type but, can't see the sides clearly. Any markings or numbers on sides or bottom of the cartridge? Where is that filter connected to?
If the unit was already in the house when you moved in, you probably don't know when the cartridges and the RO membrane were last replaced. Suggest replacing them all and sanitizing the housings and storage tank while the housings are empty.
The effectiveness of the membrane can be tested against the raw water using an inexpensive TDS meter, but membranes are relatively inexpensive and so replacing it with the cartridges may give you peace of mind that everything is clean, renewed and working as well as intended for your family initially.
The membrane will typically need replacing every 2-4 years whereas filter cartridges, 6-12 months.