new creatures in tank

Question
I have a 120gal mixed setup. a few weeks ago I started to see what looks like small barnicle on the glass and live rock. they are very small. At closer inspection they seem to have antena and they are moving. They keep increasing in number

Answer
Hi Phil. What you are most likely seeing in your tank are limpets. These will hitchhike in on live rock and will perform the same tasks as most of your snails. They move around very slowly on the glass and scrape off algae as they go along. There are some species that   if there is not enough algae available that will munch on SPS corals. The most common ones that come in on live rock are the keyhole limpets. These are usually white to a tannish brown or even black and have a distinct hole in the center of the shell. These kind are not a threat in your tank and will help rid your tank of many kinds of algae including diatoms and cyanobacteria.