Why is my garlic brown and slightly translucent?

This is a defect known as "waxy breakdown" or "waxy decomposition", and is caused by growing or storing the garlic under too hot conditions.

See http://livegpath.cals.cornell.edu/gallery/garlic/waxy-breakdown-on-garlic/, which also mentions that anaerobic storage conditions are also considered a possible cause or contributing factor by scientists.

I now buy only organic garlic. It is always typical garlic white, just as the non-organic stuff I used to buy.

However, my most recent purchase (of 3 days ago), is a different story. First use the day of purchase yielded one clove, rotted to the point that its sac collapsed, a common occurrence under long term storage, so no big deal. Other cloves I used at that time were fine.

This morning I found another much as described by OP above. This particular clove is not sticky, not particularly soft/squishy, but brown, translucent, and putting off a heavy garlic odor, as opposed to the typical clove which does not smell strong until cut or crushed, in other words, damaged in some way. So in my case at least, I believe bruising is the answer. Examination of the outer clove surface seems to support this; it shows a line of demarcation of damage, where deterioration is not quite complete on the left (also incomplete on the inside), so damage seems to have occurred on the right side of the clove and deterioration process is moving left and inward.

I'm tossing the clove out of precaution, though I believe it is probably fine.

If, in my case, the whole head had been affected, I would think waxy breakdown, but I offer this observation as another option for those coming to this site for answers.

I appreciate my new-found knowledge about waxy breakdown. Thank you rackandboneman!

It looks like it has been frozen then thawed, although that should not leave it sticky. The stickiness makes me suspect rotting as well, but I would expect you to be able to smell that. High heat could also cause the softening but I'd expect more color change in that case. (Heat could cause a milder flavor, though. Freezing usually doesn't.)

You should take it back to the store and complain, and ask them to look into how it has been handled. Be prepared to tell them how you have handled it, too, but a decent market will want to look into this because it could be a health hazard. If you bought it very recently they may also have other bulbs from the same batch to compare.

When buying garlic, the bulb should feel firm and the papery skin should not feel very loose. Over time, in normal storage (cool, dry, decent air circulation), garlic will tend to wither and shrivel and will become a bit rubbery, and it may start to sprout. It will not become translucent or sticky in normal storage conditions.

Some of my garlic looks somewhat like that after it's been stored for a year (some looks worse and gets composted, some looks better.) I've got a few heads from 2014 to use up before I start on the 2015 crop.

But that also resembles (even more) garlic that has been roasted. Allowing for the fact that nearly all recipes for "roasting" garlic are essentially steaming it in foil, with some small opportunity for caramelization, but mostly steam.

Garlic is not supposed to look like that. Its sticky, brown and soft -- I would throw that out. How did you store the bulb? Garlic does well in a dry, circulated area at room temperature.