Yeah I do have a TPU case on my MT. Its just some 10-15 small dust particles. I'm pretty sure its not gonna affect anything with the flash on the camera.
the flash is so bright i think it would have to be covered completely to be effected, you can try taking the highest resolution picture you can of something completely black and see if you can see the dust in the picture, im guessing you wont be able to cause mine has a pretty good amount built up and i still dont see any of it
but with ur case you should be pretty good, the largest area that dust can get in is the backplate, if it were to slide off in a pocket or purse dust can get directly into the phone no issue, a case, or your case, will/should stop the backplate from coming all the way off, but theres still the 3 other dust collector areas, headphone jack, mini usb jack, mini hdmi jack, altho the 2 mini ports should have a harder time letting dust inside the phone itself