Cobweb

A block with a gray/white texture which slows the player to about 10% of normal speed. The coweb can be destroyed with a sword, and unlike other plants (vines and grass) can be harvested. It is generally found in clumps of 1-7, but can come in very large groups, in over 30 all together. What makes this different from most blocks is that is does not change textures when more blocks are placed/destroyed near it, and the webs do not connect smoothly.

Credit to Pbat for finding this information in his videos.