There has been a lot of dark red slime algae growing in my tank recently.
This sounds like cyano bacteria, or blue-green algae. Maroon is a typical color but not the only color. There are other slimy, snotty algaes, but yours sounds like cyano to me.
So, I've not sure what might have trigger such outgrowth.
The tank circumstances are currently conducive to cyano growth.
The following is what I do:
1. Harvest what I can. Siphon out the slime with a vinyl hose. I filter the siphoned water through a 10 micron sock and return it to the tank.
2. Elevate the tank's total alkalinity (TA) to 4.0-4.5 meq/L while slime algae is present. I normally use limewater to maintain Ca++ and alk. I use a 2-part buffer or sodium bicarbonate to (gradually!) raise the TA higher if I have slime algae in the tank. This will also tend to push your pH up. For reasons I don't understand, higher TA has a reputation for helping to reduce slime algaes.
3. Intoduce competing algaes that are easy to harvest.
There are many other suggestions you will hear to control cyano. One common one is to do large water exchanges. If you have detectable nitrates or phosphates in the water column, this is, IMO, probably a good suggestion. However, cyano will show up when nutrient levels are relatively low. Sometimes water exchanges seem to feed the cyano rather than slow it down.
Another common suggestion is to use a red-slime control chemical sold in the hobby. These will, basically, kill the red slime. This will release the nutrients bound in the slime algae back into the tank, which is a bad thing. Also, if circumstances in the tank remain favorable for it's growth, it will come back.
Another suggestion is to increase water flow and replace old light bulbs. I don't believe that either of these directly inhibits cyano. It may shift tank conditions to favor other algaes, though.
Another suggestion is to use phosphate absorbing media. Some cyanos are capable of fixing Nitrogen, which should give them the edge if phosphate is abundant, relative to nitrate, in the tank. At any rate reduction of phosphate in the water column is always a good thing. Dr Farley showed that some phosphate-absorbing media can release enough aluminum into the water to cause some corals to decline. You might want to read his article (search under "Randy Holmes Farley) on aluminum in the reef tank before you use this.