So I am currently studying acids and bases and looked up the acidity of skin, which has a pH level of 5.5. My question is if skin is in fact an acidic substance, why is it that applying HCL or other acidic substances harms it? Does adding acid to acid induce reaction as well?

Because acidic and basic are on a scale. While your skin has pH 5.5, 1 mol/L of HCl has a pH of 0 and it really wants to get rid of the proton.

The fact that HCl burns is largely due to it being corrossive rather than it simply being a strong acid. Weak acids (that is, acids that do not donate their protons as readily) can in some cases extremely damaging to us because although they are weak, their conjugate base is very corrossive. HF is a good example of this. On the flip side, there are examples of super strong acids that are not corrossive at all.

 

Also, in furtherance to Fuzzwood's post, it is worth noting that pH is a log scale. A difference in 1 pH unit correlates to 10 times the amount of H+. (e.g. an acidic solution at pH 5 has 10 times as much H+ than does one at pH 6 and 100 times as much as a solution of pH 7.)