Because the spot appears in the same place and gets much more obvious at high f-numbers, it’s most likely dust on the sensor, not the translucent mirror.

On the A55, you can access the sensor by releasing the latch at the base of the translucent mirror and lifting the mirror carefully. For cleaning, the safest option if you’re unsure is to have a camera store or Sony service do it.

If you clean it yourself, use proper sensor-cleaning supplies: sensor swabs sized for your sensor and sensor-cleaning fluid such as Eclipse/VisibleDust-type products. Apply a small amount of fluid to the swab and make one smooth pass across the sensor, then reverse direction without lifting, using a fresh swab for each full cycle.

Be very careful with the translucent mirror, and don’t touch it unless necessary. Community advice conflicted on blowers, so the safest takeaway is: avoid canned/compressed air, and if loose dust doesn’t come off easily, use proper wet-cleaning tools or professional service.