The Muhideen Jumma Masjid is the central mosque in Marata. It was built next to the river in the center of what used to be the colonial city. It's painted white, and it's a very well renovated building compared with the remains of the colonial houses in the area. The inside of the mosque is only accessible to Muslims, but from the main bridge connecting the historic Matara with the modern part of the city, you can see it from the outside. Next door you'll see a church and a Buddhist temple. Matara is a tolerant city where everyone lives in peace and harmony. The Muslim community is mostly made up of Moors who settled in the Middle East in the Middle Ages. Their features are different from the Sinhalese, though many of them have mixed.
The Jami Ul-Alfar Mosque is the most important mosque in Colombo as well as one of its most recognizable buildings. Built in 1909 by the neighborhood Muslim community, the Jami Ul-Alfar stands out for its unique candy-stripe exterior and geometric tile-work that seems more North African than Sri Lankan.
The mosque is definitely not a tourist mosque like those you might find in Istanbul or parts of India. Rather, it's the go-to place of worship for the large Muslim trading community based in Colombo's bustling Pettah neighborhood. That being said, it is still generally open to the public but you should avoid going during prayer hours (it'll be closed to non-Muslims) and remember to dress modestly.
Visiting the Jami Ul-Alfar mosque is not an activity you should plan your entire day around, but if you find yourself exploring the sari tailors and fruit vendors of Pettah, it's worth a visit for the architecture alone.