Magnesium alloys are often used in bone repair surgeries due to their biodegradability and excellent elastic modulus, making them a promising alternative to traditional nondegradable implants like titanium alloys. However, their rapid degradation rate limits their use as implants in the body. To enhance the corrosion resistance and bioactivity of magnesium alloys, we applied an ultrasonic spray coating on microarc oxidized (MAO) AZ31 magnesium alloy, using a mixture of silk fibroin (SF) and nanohydroxyapatite (nHAp). This SF/nHAp composite embeds directly into the micropores on the MAO-treated surface without additional physical or chemical treatment, forming a stable interlocked coating structure. The effects of different spray parameters on coating adhesion and interface characteristics were investigated, leading to the development of a corrosion-resistant and highly biocompatible composite coating. Further biological evaluations were conducted through subcutaneous implantation, assessing the in vivo degradation of the samples and the surrounding tissue response from multiple perspectives. A novel concept of in vivo tissue-reactive coatings was proposed, suggesting that highly biocompatible coating materials, in the early stages postimplantation, enable surrounding fibrous tissues to closely adhere to the surface, thereby slowing material degradation. As a result, the highly bioactive MAO-SF/nHAp coating significantly enhances the corrosion resistance of magnesium alloys, reduces hydrogen evolution, promotes regeneration of surrounding tissues, and minimizes postimplant inflammation. This approach offers a new strategy to improve the biocompatibility and corrosion resistance of magnesium alloys in vivo, suggesting that the overall evaluation of biodegradable magnesium alloys should focus more on assessing in-body corrosion.