I found an article in Zaman about the namesake of this mosque:

"Ottoman Prince Ertuğrul Bey must have been a legend in his own right, as he had turned only 22 years of age when he was tasked by his father, Sultan Yıldırım Bayezid I, to try and unite all of the Anatolian principalities.

Yet he met his fate on the battlefield in 1398, losing his young life at the hands of Kadi Burhan Al-Din. Ertuğrul Bey's remains were carried back to Bursa, which was then the first capital of the Ottoman state. So one would expect to find a memorial of enormous proportions, but what we encounter is modesty instead. In our culture, sites and places of historic importance do not always have to be big -- often it is the story behind a particular ancient building, event or monument that more than makes up for a few hundred square meters of ground. Present-day first-time visitors to what has long since become one of Turkey's most famous metropolises, located in the country's northwestern Marmara region, could be forgiven for not immediately locating Ertuğrul Bey Camii, proudly standing just off from what in modern times is Cumhuriyet Caddesi. It is smaller than many neighboring buildings and attractions. Yet once there you will be glad you spotted it. His corpse was buried in the mosque's externally visible graveyard. The mosque as such survived fires and earthquakes, and so did his tombstone, although many structural repairs have been carried out on the building over the centuries. "

The mosque is the building to the right.

On the picture: The Ertuğrul Camii, as seen from the South.
The mosque has an almost square prayer hall (10 x 10,40 m), which is preceded on the north side by a porch, 10 m wide and 5 m deep. The building is covered by a tiled gable roof.
The mosque was founded in the very end of the 14th century, but had to be restored/rebuilt several times; however, unlike many other Bursa buildings, it survived the 1855 earthquake quite well.
The minaret dates from the 16th century, since the original one collapsed due to an earthquake in 1557.

Correspondent: J.M.Criel, Antwerpen.
Sources: ‘Vakıf Abideler ve eski Eserler (volume III)’ - Vakıflar Genel Müdürlüğü, Ankara 1983
& Website of ‘alanbaskanligi.bursa.bel.tr’.