What if bearded dragon keeps walking against glass?

If it's a baby, you shouldn't keep it in a glass cage. Use a plastic container of the same size with opaque sides. Most reptiles do not understand glass- it's not at all natural. It is a major source of stress. Another option is to cover three sides of your glass cage, leaving just one open for viewing. Provide at least three hiding spots- preferably ones that are low to the ground and do NOT allow you to view your pet (remember, the purpose of a hide is for the reptile to get out of view). If you're keeping an adult in a glass tank, it should be ABSOLUTELY NO LESS THAN a 40 gallon "breeder" tank (even then the dragon will want to get out into a bigger cage). Keep in mind that while 55 gallon tanks are bigger, they have less floor space than the 40 gallon, and are worse in that way. 75 gallon glass tanks are great in terms of space. They'll be hard to find, but It will be money well spent. Another option is a DIY wooden cage. It costs considerably less, is less stressful on the reptile, and comes in any size you want (you build it). Also, a major cause of stress is inadequate light. They need UVA AND UVB light (allows for proper usage of vitamin D3 which lets them use calcium. Without this they will develop over time Metabolic Bone Disease. This gives them a long and painful death. UV light has also been shown to increase feeding response, activity level, and overall health of the lizard. All this leads to less stress, and will stop your lizard from unhealthy activities such as the one you mentioned). For diurnal, active lizards like Bearded Dragons, you'll want a Mercury vapor bulb (must be changed yearly no matter if it still produces light). They are expensive, but costs like these are your responsibility to the reptile. The absolute lowest you will find a similar product is $30 (any lower and it's no good).

Good luck!