Mastocarpus papillatus, sometimes called Turkish washcloth, black tar spot,[4] or grapestone[5] is a species of red algae in the family Phyllophoraceae.

The specific epithet papillatus ('with papillae')[6] is due to the nipple-like projections on the female gametophyte which can give the texture of a terrycloth washcloth found at a Turkish bath.

It is instead native to the northeastern Pacific Ocean and commonly can be found in California, Oregon, Washington, British Columbia, and Alaska.

[14] Mazzaella affinis thalli looks similar in shape and color to male M. papillatus, but are smaller and grow in thick mats and not as individuals.

[14] It also can be confused with other species with Petrocelis phases (see below), such as Pikea, other Mastocarpus spp., Ralfisa, and Hildenbrandia, thought the last two are thinner than the Turkish towel crust.

[16] Grapestone is edible[4] and can be cooked in soups or stir-fries but care must be taken as overcooking can turn the plant mushy and gelatinous.