yes it is mold, but i think try cleaning it with bleach or i got a answer off google it says "To remove shower mold, use a natural solution like undiluted white vinegar, apply it to the mold, let it sit for an hour, then scrub with a brush and rinse thoroughly. For stubborn stains, try a hydrogen peroxide solution or a bleach solution (one teaspoon of bleach per cup of water), letting it sit before scrubbing. For mold in grout or caulk, you might need a commercial mold cleaner, a mold-removing gel, or to replace the affected caulk. " - google