Your Hedera Ivy has a very serious spider mite infestation. If you rub the leaves between your fingers they will feel gritty. The spider mites are very tiny and make very fine webs. Hedera Ivies are among their favored hosts..
Fill your kitchen sink with soapy water. Them, invert the plant, holding it in the pot with one hand and swirling the plant around in the soapy water. This will ensure complete coverage of all leaf and stem surfaces, which is important.
The mottled leaf damage is permanent, but if you successfully treat the mites, new growth that comes in should be healthy.
Spider Mites tend to attack plants that are under stress. Yours is stressed because of the way that you have watered. When the top half inch of soil is dry, then water thoroughly so that all of the soil is saturated. It will take at least a week for the top half-inch to dry out again and need water. Also, keep your Ivy close to a moderately sunny window. Hevera Ivies are not low light plants.
Finally, fertilizer is not medicine and is intended only for healthy plants that are growing vigorously and depleting soil nutrients.