Just to add to the above. Injured/harassed fish are stressed. Stress is one of the major gateways for disease and parasites. This means your cory is more at risk. Further, fish which are normally out and about which suddenly go into hiding are usually doing so as a defensive measure. They hide from any harassment etc.
I cannot say that your cory has no disease issues for sure. I do know what has been said about tiger barbs is correct. They can be nasty little fish and the best way to blunt that aggression is to keep them in bigger numbers. There is one disease which can manifest in the way your fish looks. This is columnaris (Flavobacterium columnare). It is often called by several common names, cotton mouth, saddle back (because of the whitened area at the base of the dorsal).
This disease comes in a variety of strains. The most virulent kill so fast one rarely sees any symptoms. This is clearly not the case with your fish. Other strains are less virulent and can be treated. However, it is important one not medicate a fish unless one is reasonably certain of what it is they are treating. Please do not conclude you fish has columnaris at this stage. What I do suggest is water changes and closely monitoring the cory. If more white splotches appear, if you can spot what looks like fungus around the mouth of the fish, then you should conclude it is columnaris and it must be treated.
Treating columnaris is challenging because it usually needs antibiotics and this disease has developed resistance to the most commonly used- amoxicillin, doxycycline, benzylpenicillin and tetracycline. If you can determine this is the issue, I can point you towards a potential combo-treatment that has proved to be 70% effective, The problem is it will likely cost way more than you will want to pay. You would need a three drug combination and that can cost $50-$75 (maybe more). This will treat way more fish/water volume than you need to medicate, but one cannot buy meds in tiny amounts.
For sure, I would figure a way to get rid of the tiger barb- i.e return it to the store or give/sell it to another fish person since you cannot up the numbers significantly. Please do not kill it. Fish are not evil killers, some have just evolved to be more aggressive. One cannot change the inherent nature of a fish. You cannot teach a piranah to become a vegetarian.
